I might be missing something on the multislotting of modules into one key because it is not working consistently.
I have a virus weapon and a drive dissipator weapon both grouped on left mouse click and it seems like only the primary weapon gets fired the majority of the time. Once in a while the drive dissipator will fire, but that's about it. I'm full on energy so that's not the issue. I can verify this by both watching the debuffs on the enemy or by expanding the control group and watching for the cooldown animation to play on the second weapon. It only triggers maybe every 3-4 shots or so.