Are there any car seats that fit in the Snap Ultra Duo without adapters?
Or do we have to buy adapters?
Lastly, are the car seats on the same level, or are they high/low positioning?
There are no car seats that would fit the Snap Ultra Duo without adapters, I'm afraid. The pushchair is not wide enough to accommodate both side-by-side, so it would not be possible even if they tried to make some sort of attachment system on the belly bars or so.
The adapters - you will need a double adapter set here - are made so that, yes, one capsule is slightly lower than the other so that the whole bulk of the stroller with two car seats is not insanely wide (if they would be absolutely side by side here, they would, similarly to a Bugaboo Donkey, be wider than a normal 80 cm doorway is)
I am attaching a picture of what you'll be looking at with two adapters sitting on the frame 😊 The car seat fitting this setup will be most usual-looking capsules as it's just put on the adapter frame and secured by the strap.