I got my marine to shoot his Gauss Rifle and Marauder's 'nades at the same time by adding the wanted weapons to my Marine (Data Editor -> Marine -> Combat - Weapons). After that I went to edit the Marine's Gauss Rifle (Weapons tab -> Marine - C-14 Gauss Rifle -> Weapon - Options, and unticked "Linked Cooldown").
If there are any better ways, someone should post 'em here or make a tutorial. Hope this helped!
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.