Probably best to buy these things as a complete set. The forward and rear props are matched together, and I think, balanced together. Putting out-of-balance props on to the drive legs, will lead to early bearing failure.
Good one GC1 - It took me a second or so to match your comment with the picture that John1 has as his icon! Made me laugh for a while. I think you may need to retake your piccie John1, or tell us why its sort of folded!
...think so long as each prop is in balance, then that's ok, unless one has been customised, e.g. by hitting the bottom and then being repaired in a way that made it different to the original.
The most important thing with duos is "perfect pitch". If the front prop is even a slightly different pitch to the back one, then there will be a huge load on the lower bearings. The safest way is to buy them in sets, but if you're 100% sure that the remaining (good) prop hasn't been modified, then a replacement for the other should be fine.
I took my props off to clean them and noticed when all the gunk was removed, the front is a C6, the back is a C5.../forums/images/graemlins/shocked.gif
Know a cheap source of a pair of C5's by any chance?!!!
(C= stainless, A= aluminium for any non-grandma's not aware of egg extraction technique)