They're more or less ineffectual played together, it's just all too... clunky and oversized, if you understand what I mean!
Seeing Rom, Zlatan and Fellaini together just depresses me somewhat as a United fan - yes, they're different players, but the way we're playing has such a focus on crosses into the box that the 3 of them are often just standing around in the box together so statically.
And if they're not doing that, Rom is out-wide (Romelu Lukaku OUT-WIDE...) doing his turn at the United 'square peg in round hole' thing that for some reason this club seems to almost insist on - remember when Mourinho turned up, it was 'specialists in their position'?
Well now it's Rom out-wide, which is literally pointless.
So in short, Zlatan and Rom together? No. Rotate them and use each other to provide competition for place.