Had a good chat with chap at the NNS on Saturday who is looking for a trailer that could carry his twins. Suggested he check out the forum for useful advice. He's keen on a chariot which is apparently a Canadian make (didn't know that!). I said we'd had good luck with the bike coop trailer, but that it didn't convert to a stroller.
I also mentioned the Helios, which is fine until they are 3-4 ish, I guess.
One benefit to the trailer is obviously being able to move it between bikes.
Any other ideas?