I actually have reloaded weapons while holding another. It's pretty manageable. Shame though, that would have been an interesting loadout with the conservative  tactical usage of bounced grenades and EMP's, and skilled control bursting of the automatics. By the way, I get the realism thing, though I'd like to point out that many real life weapons are capable of mounting 40 mm grenade launchers under the barrel on a rail. Also the grenade launcher in the game was nowhere close in power to the rocket launcher or turret. Hell, it was weaker than a regular frag.