Since I have not seen paired weapons in game so far, I decided to drop here an idea I had for some time.
There is one-handed ranged weapons, that distinguished from two-handed by adding artificial delay for switching to the last. What if we go creative way?
If you have, say, chainsword in melee slot, and laspistol in ranged slot, you can switch between chainsword/laspistol-only to chainsword+laspistol by tapping melee weapon button when you have laspistol equipped and vice versa. So this way you will actually have three set of weapons, kinda. You can tap on melee/ranged weapon button while dual wielding when you want to use single weapon.
Most important, this way you will be able to wield any possible one-handed weapon combination. (There is issue of what traits/stats will work, but I’m sure it solveable.)
I think paired weapons will work similar to Saltzpyre’s Rapier and pistol, with shot on alt attack button.
Oh, and you can use the same mechanic (tapping the same weapon switch button which already equipped) for switching firing modes for ranged weapons and adding stances for different melee attack patterns.
P.S. Tapping the same weapon button which already equipped to switch to dual wielding may be better, I think, since tapping melee weapon button while wielding ranged to block the enemy strike will switch you to dual-wielding, and that may be detrimental.