If you lose your original ball, your provisional becomes the ball in play (with the penalty stroke). If that is lost as well, you again have stroke-and-distance. The rules don't treat it any differently than if you'd walked back and hit another ball OB or whatever. So in this situation, you should hit a second provisional, which only becomes in-play if you lose both the first two. You can continue hitting provisionals until you are sure you haven't lost a ball, technically speaking.
Also, normally you get 5 minutes to look for your original ball, then if not found, 5 more minutes for your provisional. However, if they're in the same area, then you only get 5 minutes to find either of them.
Finally, if you do hit multiple provisionals, be absolutely sure that you can identify conclusively which ball is which. If you can't, even if you find a ball, you cannot identify it properly so it is lost and.... back to the tee box.