Unlike the "extra arms" attribute and all its friends, additional ranged weapons do not grant additional attacks. If I have Eight arms with a blade [or not] in each, I can get 2 [normal two-weapon-wielder] attacks, plus four [and these could be swifts too], all as part of the same action. We're looking at a potential maximum number of hits equal to 6*WS.
If instead all those extra arms were filled with pistols, you get… your normal two-weapon-wielder attacks as one action. And as a reaction you're allowed to use a mechadendrite.
That's it. A Weapon MIU doesn't even technically count as a one-handed weapon, though if you're already sporting weapon-bracing or slayer arms, it should be a very "yeah sure whatever" case of asking your GM. One game I was a part of, I was allowed to use two two-handed weapons: One on the MIU, the other in-hand. Of course, given what the melee tank was pulling off [a WS70+ Sb6 guy with x3 unnatural strength when using power-fists] it was kinda needed.
If you want to mount a weapon on a character, you have the following options other than the dendrite, which is kind of a seperate affair:
-Weapon Bracing [perfectly compatible with the actual MIU, but a different implant of its own]
-Armour upgrades [Concealed Weapon and Tool Mount]
-Slayer Limb Mutation [marginally quite heretical, very slightly blatantly obvious]
-There's also Recoil Gloves, which let you use a basic weapon one-handed with no penalties, though its not mounted
-And failing that there's also Carbine models, which have a -10 instead of -20 penalty for using one-handed.
All of these save the MIU allow you to use the weapon one-handed, meaning you could technically be dual-wielding them.
Finally there's one more thing the character can do, which allows for extra actions up to a point: Get himself a minion, or a servitor or servo-skull or whatever you wanna call it. Or a vehicle. Even a simple bike could mount a heavy weapon, which you can then MIU uplink with.