I always go with the premise that the other half is generally irrelevant and in the scientific sense, the other half is the recessive genes/traits that have been overridden by the dominant ones that manifest as the characters outward appearance and racial features (as per the race details in the PHB).

So fluffwise it doesn't matter if you're a half orc/half human, half orc/half dwarf or half orc/half pint of ale, if you choose half orc as your race you look like a half orc and get the racial features as such.

As far as racial features goes, I don't allow mixing. If you want something thats specifically for elves/half elves, then you choose elf/half elf for your race at character creation. I might allow it for a throwaway game or one shot if you can convince me as to why you should get it, but it opens up some avenues that may make things overpowered and unsporting for other players.