Buying a Tandem Double Buggy
If you have twins or children similar in age, a tandem double buggie is a great choice. They're narrower than side-by-side buggies and are ideal for maneuvering narrow spaces like entrances, on public transportation and rocky terrain.
Some transform into travel systems that take 2 carrycots or car seats. However, pushchair twin are heavier and longer than single pushchairs and may feel less stable as they bump up kerbs.
Convertible
If you're looking for a convertible stroller that converts from single to double or to add an additional seat to an existing one, there are several options available. The Vista V2 from UPPAbaby is a large, gorgeous stroller that converts easily from a single model to a double by incorporating the rumble seat (available separately). It is more expensive and less maneuverable because the second seat is positioned in front of the first seat. The Nuna Demi Grow is a similar model with great seating versatility but isn't equipped with storage under the seat when used in doubles mode.
Other options include twin buggies with all-terrain wheels as well as joggers that have fixed wheels that can be used to carry two kids. They are generally more expensive, but they're made for long-distance trips over rough terrain, and may have a higher resale value.

Reclining the seat independently
If you're looking for a double buggy that is able to carry children of different age groups, then you'll want one that has independent seat recline for both seats. This allows your older child to rest while the baby is in the bassinet, and vice versa.
Consider the size of your pushchair and consider the ease of you to navigate through doors and on public transportation. If you have extremely narrow doorways then it might be a good idea to consider a side-byside pushchair, such as the Mountain Buggy Duet which has the advantage of having a footprint that is similar to a single buggy, however it can use two seats and car seat adaptors.
Tandem buggies generally are smaller than side-byside models, and aren't as easy to manoeuvre around kerbs however, they can accommodate a wider range of car seat/car seat combinations, making them a good option for twins or siblings of different age. They are more expensive than single buggies to double but they're well worth it if you intend to keep it for a few years. They also hold a good amount of value if you decide to sell it when your kids have outgrown it.
Another thing to look for is whether the seat backs can recline independently from one another since this will allow you to place your child in the best position for naps or watching. This is particularly important if you have a young child in the rear seat, who will be inclined to lie down and take a snooze while the older child wants to sit up and look around their surroundings.
Certain brands have a unique design that allows the back of their vehicles to fully recline into a position that is nearly flat, which is ideal for infants. For instance, the iCandy Orange has raised stadium-style seating and can recline both the toddler seat included as well as the optional RumbleSeat to multiple positions, so your kids can have the option of deciding where to sit, as well as offering an excellent UPF 50+ canopy that has windows that can be seen from afar.
Wider than twin buggies
There's no need to have twins to benefit from a bigger double buggy - they're ideal for siblings of different age groups. They allow you to go about your day without worrying about your children touching each other. They're typically a little larger, with one seat sitting behind the other, and typically offering a fully-flat recline for infants (or a more upright position for toddlers).
Mountain Buggy Duet is a popular option. It has the slimmest design of the two-in-ones on the market, and a footprint like the footprint of a single stroller. It also has an adjustable handle, making it easy for parents of different heights to push as well as a handbrake that is useful when tackling difficult hills. It is suitable from birth and is compatible with a variety of car seats and carrycots, meaning it is a great travel system.
Another alternative is the iCandy Wave, which has the convenience of a single click to fold and can be used in mono or duo mode. Its clever frame expands widthways to allow for a second seat and can be used with up to two carrycots or car seats so it's an excellent option for older siblings that want to sit in a group and enjoy the view. It's slightly larger than other models, but it still isn't heavy and has a soft suspension for kerbs.
There are a lot of 3-wheeler tandems inline available - check out Phil and Teds inline models, or iCandy's Peach and Pear buggies, for instance that provide stability and quick steering. They can be folded into one buggy in case you have a small space at home or you need to transport it from your car to the bus stop.
It's worth bearing in mind that a wide double buggy will be heavier and bulkier than a single buggy that is smaller So make sure you're getting what you need before purchasing. You'll have to get it off and on trains, buses and subways regularly so it needs to be simple to use and manage. You may need to juggle the car seat in and out when taking your child to school or to nursery.
The best time to start is when you are born.
Whether you're expecting twins or have a second child after your first, you're going to need to find a stroller that can accommodate the whole family. Double buggies tend to be slightly heavier, bulkier and cost more than single strollers, however they are also more maneuverable in narrower doorways or around obstacles like curbs. Many double buggies can be used with piggyback boards (available from the manufacturer or as an add-on). This lets you transport three children at once.
Tandem pushchairs place one child in front of the other. They're a good option if your children are of similar ages and are able to get along. They're not 100% secure. Siblings sitting side-by-side can punch or kick each other, and children of different heights can fight over who is sitting higher. A tandem high-low model can solve this issue by elevating the seat towards the rear, which will prevent your child's height from being on the bottom and not getting any view.
The iCandy Wave is a great option for parents looking for an easy-to-use convertible double buggy that can fit a newborn into a carrycot, and an older child in the main seat. It features an innovative frame that increases in size when you add the second child seat. It folds flat for storage that is compact. The cushioned straps, buckles, and seats are all of the best quality we have encountered in our tests. The canopy is large in size with UPF 50+ protection, and the window is peekaboo. Both seats can be reclined independently.
The only drawback of the iCandy is that it's a little on the expensive side, but a quality tandem will last a long time and will retain its resale value, which means you'll probably recoup some of the cost in the long run. The iCandy doesn't come with the same seating arrangements as our other top selections. However, it's an ideal choice for families that don't want spend lots of money.