A tandem double stroller puts one seat behind the other. The frame is longer, not as wide as side-by-side.
Measure length first: the lift, the hallway, the car trunk. Then check how each seat is used. The front and the back can differ.
From birth needs a complete setup for each child who needs it. Read both sides of the product page. That may be carrycots, or the listed from-birth seats. Do not assume they match.
Then read what the package includes. Confirm extras on that page, not from the other layout.
If you need two seats next to each other, compare side-by-side double strollers. All two-child frames are in double strollers.