I'm playing on 20 not 21, but in my experience there's nothing preventing you from labeling them both as an ace. The only issue is that one of them might have a drop in his morale, but I would expect that it would likely be fairly minor.
You can promise anything that you want in contract negotiations and the morale is the thing that gets effected the most. The more that you stray from what you offered, the bigger a deal it becomes, and the more the player's on field performance is effected. So if a guy expects to be an Ace and he's in the bullpen, he might have a really big drop.