I've had lots, ds was born when dd 3 was two, and I also have twins :
emmaljunga - very expensive, heavy and takes up fair bit of boot space, also very wide and won't always fit through doors. But this is still my fav, as it is really comfortable, lovely to push and reclines fully to sleep.
jane powertwin - I think this is the best tandem as it doesn't seem too long. Folds do3wn fairly compact too. Only prob is the front seat doesn't recline very much, and with a tandem the baby in the back can't see out, which is why I also have the pram.
we2 combi - really nice, quiet easy to push, very lightweight goes through most doorways. Lots of handy pockets, folds down really small. But, all the joints qand plastic seem to make it weak and I smashed the bumper bar on mine, and it felt flimsey.
maclaren twin traveller - hated this. Too heavy, horrible to push, very narrow seats. Only used it twice then sold it on.
graco duosport - Liked this one too, very lightweight, folds small, reasonably cheap too. Seemed robust enough. This lasted me until dd was old enough to walk.
cossatto duolite - I liekd this, but the seats were small, and the dt in the back couldn't see out, so we didn't keep this neither. The back made a nice coocon for a newborn tho, and might be ideal for different aged siblings.