Well, obviously first off, odds of that are basically nil, seeing as each of them is in line for a bajillion dollars, and the cap can't sustain both of them.
That being said, the answer depends on the team they sign with, the system, and the QB.
Lets say they sign with Denver. There's probably no drop off. Manning has 5k yards to distribute, and in fact, Emmanuel Sanders, Denver's #2 guy actually did better than Dez and was 5th overall in yards (Thomas was 2nd, Dez was 8th).
But let's say they both sign somewhere else. Maybe Pittsburgh, with a QB like Roethlisberger, good enough to take full advantage of one receiver but probably not good enough to take full advantage of two.
Which one of those two succeeds probably hinges on who Big Ben prefers and forges a strong relationship with. The other probably does alright, but likely drops a fair amount.
It is possible that a QB like this might go equally to both, in which case they would indeed both drop, but QBs tend to lean towards one receiver over another when there are two solid ones on a team.
Lastly, imagine they both go to a team with a weak QB. Neither is likely to have high fantasy numbers if the QB can't take full advantage of them. Which one gets closer to their full potential, like with Big Ben, is based upon who the QB/system prefers.
So, the rough way I figure it'd work:
Great QB - No likely drop off on either
Good QB - One has a drop off (most likely), both have a small drop (less likely)
Bad QB - Both drop