I vote for one of the doubles. Your 2 YO may want to walk around for part of the time. If you bring 2 singles, then one parent will have to push an empty single and try to hold the hand of the 2 YO (or one parent would be trying to push 2 singles while the other is chasing the 2 YO). So, no matter what, I say to bring a double.
I wouldn't bring two singles - you guys can always switch off if the weight is too much going up hill. For my kids I would do a sit and stand because Maggie refuses to really sit in a stroller seat and she could still sit on the sit part of the sit and stand. if your 2yo still like sitting in a real seat and you plan to be there a long time then I think either option works out fine.
Re: going to the zoo, stroller help poll