The grammar patterns of Courts Martial, Judge Advocates General, etc. would seem to agree. In example, those who pass flatulence would be "gas passers" or passers of gas, just as passers by, which is short for an entire phrase "passers by the side of [implied or mentioned object]" is different. However, "someone else" appears to hearken back to a more Germanic form of grammar, rather than the French Norman with its Latin influence. If this is the origin of the phrase, then using the entire phrase as a single noun or idea would be appropriate. In this case, where both words originate from the Germanic, it would be "someone else's". The Germans frequently abbreviate such phrases where they become excessively long, but in their original were written as one word using their cursive. In school I studied French, Classical Latin, and German enough to become aware that our aggregatenous language has so many exceptions because of those origins. (I have dabbled with Gaelic which is as far as I can tell the source of split infinitives.)