I'm not an expert, but from my experience (which is limited), I'd say you have a 50/50 chance of success, assuming both are still juvenile.
I don't have any experience with snowflakes, but my first two fish were ocellaris clowns (orange/white & black/white) and purchased in the same manner. They were roughly the same size (1" or so), and seemed to be doing well. After about 6 months the black/white ocellaris died. I don't know if it was killed by the other or died of natural caused, water quality issues, etc...
I tried pairing with another (noticeably smaller) ocellaris and it died. I assume it was killed as it only lasted a short time.
I gave it one more try, with another black/white much smaller ocellaris and they seemed to have paired up nicely for the past 10 months or so.
In my opinion every fish is different. There is really no way to predict what is going to happen until you get them home and observe. I'd say your best chances are with two very young juveniles or two of noticeably different size.
Again, just my .02, I'm no expert.
You can always ask your LFS to try and pair them for you, but they will probably want to charge you more if it's successful, rightfully so as they are assuming the risk at that point.