My and my brother have Gamertag files in my Xbox, and it never matters who downloaded what, we both are able to play it.
So if one of use gets the Premium membership, would we both be able to use the maps downloaded? I'm pretty sure we both could, but my brother is pretty sure we couldn't, so I feel I should make sure.
I'm assuming the Membership gives you redeem codes to downloaded it off the marketplace for free. You can't play the maps if they're not downloaded, and any map I've ever downloaded we're both able to use since both files are on this 1 Xbox, so I don't see why it would be any different.