It depends. If one of them says you need "goblin" or "gnomish" engineering then you may have made it before, but you wont be able to use it the 2nd time around when switched that specialization.
However, if it doesn't say that but just one of the specializations was needed to make it and every engineer can use it, then yes you will be able to have several such items.
It's a good change for those who just decided they chose the wrong one or want to change for some other reason. But keep in mind, you have to re-work the tradeskill from 0 again.