Sits one on top of the other I find easier to manoeuvre and get on and off the bus with 😊
I have a side by side one, a bugaboo donkey, but it fits through single doorways. My boys are both tall so I personally didn't want one constantly kicking the other, or bored as they can't see the world. X
I don't know what the buses are like in your area, but we've recently had some new ones be rolled out, and there's only one space in total for wheelchair and a pram and I reckon you may struggle getting a side by side one on those kind, although I suppose it depends what the buses are like near you. Sorry I don't have any actual experience with the double prams though x