Side-by-side doubles are usually easier to load (both kids visible, both accessible) but can be too wide for narrow shop doors, some buses, and older house hallways. Measure your front door and your car boot width before falling for one online.
Tandem (front-and-back) doubles fit through standard doorways but the rear seat often has a worse view and less recline, which matters if your younger child still naps a lot.
If your children are more than about 2.5 years apart, a "second seat" add-on to your existing single pushchair is often the cheapest and most compact route — your toddler gets an occasional ride, and you’re not committing to a permanently wider buggy for years.
Whatever you choose, check the maximum combined weight and the seat weight limits individually — it’s easy to forget a bigger toddler may already be close to a seat’s limit before the buggy is even a year old.