The problem is you need it big to fit stuff in for 2 but you want it managable too. The Ju Ju Be Beprepared is good. Large enough but not too big. Or the Babymel X2 is great cos they can both have their own bag!
I have a ju ju packabe and its brill. Have used it non-stop since it was bought for me a year ago (they are expensive!) and DS is now 2.6 - its great for his juice/snacks/nappies/wipes/change of clothes/emergency toys etc etc. DS2 is on the way and there will still be room for the extra nappies etc.
I specifically hunted for a rucksack style bag once DS was older because I spend so much time running after him I can't be doing with a shoulder bag slipping off constantly whilst trying to grab an evasive little hand...
I just bought an OiOi large messenger bag. Its lovely and stylish and is very roomy. DD should only need a couple of nappies ( wipes ill have anyway) few toys and snacks so lots of room for babys stuff.