J.G.
More than likely, there's no reason not to. The concerns against that have to do with allergies and sensitivities. Parents are encouraged to stick with one brand for a little while so that if baby does have a problem it will be fairly easy to figure out which formula is the culprit.
We used all of our Similac samples, then all of our Enfamil samples, then purchased the store brand. I asked our ped about that, and she assured me that, unless baby has a reaction, the store brand is just as healthy and safe as the name brand. Saved us a lot of money, as the store brand tends to be just over half the price of the name brand.
If you know baby does fine with Similac, go ahead and use the Enfamil. I would use it for at least a week just to be safe. That way you will feel confident that baby is ok with Enfamil, too.