I think the main thing to look at in tandem prams is the width - if you go for a side by side, does it fit through a regular doorway. If it doesn't, it can be highly frustrating and using it for most things tend to be a bit more difficult.

Three wheeler side by sides are not recommended for babies of different weights as they can tip easily so check on how well balanced it is.

That's probably my best advice. I would also try and put it in and out of my car before I decided on anything. Some of the baby shops are really good like that and will let you take prams out to your car before you buy them so you can practice folding them up and lifting them into your boot to see how well they fit and how easy you find it. I highly recommend doing this with any pram that you buy just so you know how it will fit in your car.

Happy shopping