I bought a mei tai, they're my favourite as really comfy and not too complicated. It's a beauty as well, not sure how to get the picture up but i'l post one of me wearing it when its here lol.
Theres a facebook group for "slings and things" (I used my mums account to have a peek as im not on facebook) but theres tons of advice on there and loads of preloved ones for sale. Someone asked yesterday about tandem wearing twins actually.
I'd have no idea how you'd go about it with twins but I have seen enough pics on this page to know it can be done