Kristy - there are currently only TWO twin prams on the market (as far as I can find!) that take a toddler seat. One is the Valco the other is called the Adventure Buggy. It's made by the adventure buggy company. If you google that, their website will come up and you will be able to see some pics. The adventure buggy has a more comfy looking toddler seat, I think. Although I haven't seen one iRL yet - just pics online. They are the ones that have a new triplet pram out too so worth checking out. The toddler seat is up high - kind of on the handlebar so the toddler faces you.
I am still deciding between the two. Beware though - they are both super spendy! The new valco is about $1000 including the toddler seat and the adventure buggy is about the same (although you can get deals through AMBA). Remember whatever you buy, to tell the store attendant you are having twins - most baby stores give 10% off if you mention you're having twins.

HTH!!