I think it depends on a few factors - Like the age of your LO's, weights of them etc....
I've had to plum for a tandem because when I put Ky in a twin with his cousin - He WILL NOT leave her alone and he really annoys her, even though he is just rying to hold her hand or kiss her etc.....I thought I best have the baby where he can't reach her because although he would never intentially hurt the baby, he is boisterous. By the time the baby is a bit heavier, I'm hoping I can go back to my single pushchair with a buggy board.
I'll let you know how I get on!