They can wield the six weapons at once, and use them to defend and so on (which would work pretty well against other weapons as things stand), and attack with any single one they want while the others are defending (which leaves five defending weapons, which is good) -- or they can attack with a few weapons simultaneously at a penalty and defend with four, etc. Why they shouldn't be penalized for simultaneous attacks is more complicated, since the arguments for the penalties have to do in part with putting your body into things and using stance points to anchor strikes and so on, and that is partially independent of the number of arms (though arms coming from the same direction could be anchored the same way, perhaps). I haven't done anything with that. Hydras have special flags for their bite attacks because they can be performed simultaneously without penalty, since the hydras necks are independent and the body isn't involved to nearly the same degree.
But, yeah, the larger point hasn't been addressed for wielded objects... if you had some sort of hydra-neck-limb independent of the body that should be able to whip a weapon around independently, it can't be done yet. I went with attacks since it is a higher resolution than the body part, and you can imagine scenarios where a single body part has an independent attack, a regular attack, and a bad-for-multi-attack (a tag which is used for kicks now). At the same time, there does need to be some sort of bp-wielding-item handling -- and it has to play nice with the item flags, since items also have the hydra-style independence flag available, etc.