I'd probably use the appropriate combat skill for the weapon coupled with Willpower as others have suggested, then I'd do one of the following:
- Increase the difficulty of landing the attacks by 1 from whatever it would normally be, or
- Upgrade the difficulty, or
- Add a Setback die
After all, it should be challenging to pull this off, even if I think that it is absolutely covered by the final Move Control Upgrade.
If simply committing a Force Die per weapon being wielded in this way seems too lenient, I'd say that the character would have to activate the power as normal, rolling their Force Rating along with the modified combat skill check--obviously, if they fail to roll light side pips (and decide to not tap the dark side), the strategy doesn't get off the ground. If they want to wield multiple weapons, they'd need to have Magnitude Upgrades and the pips to activate those as well. Possibly allow Range Upgrades, but if I did, I'd definitely make the combat checks increasingly difficult. I like Donovan's suggestion for the rules governing attacking with multiple weapons.
Then, I'd probably require the character to commit as many Force Dice to the task as they like, rolling whatever dice are committed each round with their modified combat check. I know that this isn't the way the commit mechanic works, but I see it as a combination of the idea of concentrating that the commit mechanic conveys, as well as still allowing for the possibility of that concentration faltering. This would allow someone to activate 3 weapons, but possibly drop one or two of them partway through the fight due to a poor roll.
I'd say that, if simply committing the dice, without having to keep rolling for the power, I'd probably increase or upgrade the difficulty of the combat check, but if having the commit/reroll hybrid, I'd simply add a Setback to the combat check.
A Despair should probably result in the weapon being dropped, or thrown.