FMG9 Akimbo + Ext. Mags is a hack.
USP.45 + Ext Mags... I believe that doesn't matter as it's a regular attachment? Unless you mean USP.45 Akimbo + Ext Mags in which case it's also a hack.
No worries. people have decided to completely hack the game and turn the FMG9s into Primaries therefore making them elegible for Proficiencies (2-attachment) and then adding both Akimbo+Ext. Mags.
These hacks are just hacks... nothing more.