I actualy like the idea of the multiweapon system, since I can mix in my favorite weapons from the game, however what I think it lacks is utility.
My idea is to make the Multiweapon be like more the weapon switching from the Hero class in base PSO2. That means when you add a multiweapon, your weapon unlock a new weapon swapping Photon art, and the photon art will vary depending on what multiweapon you added.
Like for example, if you are using any weapon, and you multiweaponed it with another melee weapon, you will get a Gap closer photon art, you can just simply click it to use the photon art, or hold it to switch to the different weapon, and the secondary weapon will also gain switching photon art. Lets say that the main weapon is an Ranged weapon, so if your 2nd sub weapon is ranged, it will have an ranged (Similar to how the Hero sword can shoot those plasma pellets) switching photon art.
The visualisations of the Swapping photon art can be vary and different depending on what weapon class you are using.
And you gain 6 slots for both weapons (6 PA slots for your mean weapon and 6 slots for your sub weapon, the weapon swap PA's will swap between weapons, unlocking the PA weapon pallete of your different weapons.)
Also, both weapons should be visible when using multiweapon, the one weapon you are correctly using, and your secondary weapon that should be holstered.