i like the idea but there may be problems like-
-the player may not have both the games, well but its kind of an advantage because the player may get curious and buy the other game too just to see the bonus
-if you are going to use codes then many will just google for it and then the main concept of unlocking may get spoiled
but well, in both cases the developer will have the advantage(his/her game will $ell )
-if the games are being created by more than one developer then both have to be in agreement and IF for some reason one cannot include a feature then the other will also have to remove the bonus and everything will get kind of mixed up
at the moment I haven't thought of all the outcomes of this idea but it may be a problem for those who don't want to collaborate