If a character has two power fists can they use other weapons with the power fists? Like bolters or power swords or something else if they don't have the fists activated to attack with them as melee weapons?

Given it's possible with terminator armour whose fists should be standard power fists...?

Yes.

Only one of a terminator's fists is a power fist and that's only standard on loyalist tactical terminators.

Two power fists is odd, since you can one hand tote a basic or heavy weapon in terminator armor, but possible. Your manual dexterity would probably be shot to hell and don't expect one of those oversized fingers to fit in a trigger guard.

You might be able to customize a basic weapon for use, it would be odd, and you'd probably have to drop it to go into melee. Mounting specially refitted basic weapons on the BACK of the power fist itself is rather popular. Get an integrated ammo supply though, you aren't going to be reloading for @#$%.

Terminator Suits do not have a standard power fist. It is only a standard wargear in many configurations.

Also a Power Fist can not manipulate other weapons. You might use it to stabilize a Bolter in it though that is not that impressive for a simple recoil glove (standard in decent power armor) allows you to fire it one-handed in the first place.

A Power Fist is simply to big to fit into the fine manipulation of most weapons and even for swords it might be to slow and cumbersome to use them effectivel. Do not forget that the power fist is controled by an inside mechanism around the fist. The fingers of an Astarte are not actually in the fingers of the fist. This prevents any elaborate melee-weapon handling for you simply cant use the fist like a regular hand and so I would pose a minimum of -20 difficulty.

Powerfists canot be used to wield other weapons. Not only are they to cumbersome and have no finer motivation, the powerfist is also sheated in a type of power field, so anything grasped in one would be crushed and fried.

If your GM allows it you might be able to attach wrist mounted weapons to them. Marneus Calgar (Ultralmarine head-honcho) has twin powerfists with underslung bolters. But those are kind of unqiue weapons.

I see no real issue so long as you don't have the power field activated. Notably, the rules don't actually prohibit it (as they do with the chainfist or with lightning claws), and while the fingers may be larger they're probably not significantly larger than Terminator fingers are already. (Hence the business of Terminator armor already being limited to certain weapons.)

As to the business of using waldos rather than your real fingers, well...take a look at the opening cinematic for Starcraft 2. (Check 2:10 and later for the relevant bit.)

Little difference is that the SC Artstyle and Weaponry was made to be used that way.

Power Fists were not.

The setting actually makes it a pretty big deal if there is a Power Fists with other armament mounted on around.

Difference between a power Fist and regular terminator gloves is that later are still flexible and unlike a power fist have no fixed wrist joint. And you need that to fight with something like a sword and that is not just pushing faces with.

As for the finger size there is an incredible difference between terminator and power fist fingers (Those are humongous). Just take a look at official miniatures and artworks.

You wont even be able to hold a regular Power Sword in such a fist without breaking the guard and touching the blade itself.

Mechanically, you're in the clear, but I have to agree with the Moep in regards to the style impact. It just does not feel right if we consider how huge a power fist looks . If everyone on your table or at the very least the GM don't have a problem with this, go for it, but personally I would consider this rather cheesy.

If you absolutely have to add weapons to your weapons, at least expend the influence/resources to properly bolt them on top like with the Grey Knights' wrist-mounted storm bolters ... or the chainfist, in a way.

Black Crusade does not specifically state that you cannot use other weapons with a Powerfist.

That being said, Lightning Claws and Chain Fists do state that they cannot be used for anything but the most basic manipulative tasks; in addition the Chain fist discription before stating this is:

"Each gauntlet is as bulky as a power fist and cannot be used......" Pg. 163 Core Rule book, Black Crusade

which would indicate that powerfists would also be to bulky for weapon wielding.

If you have a look at the Deathwatch Core Rule book :

"It replaces the normal glove on Astartes power armour, and imposes the same weapon restrictions and manual dexterity penalties as a lightning claw." Pg. 152 Core Rule Book, Deathwatch

Which leads us to the Lightning Claw description on the same page:

"their bulk makes them impossible to effectively use ranged weapons of any class other then thrown....In addition, the poor manual dexterity penalty increases to -20." Pg. 151-152 Core Rule Book, Deathwatch

Now technically RAW, you can wield a melee weapon in a Power Fist but you would be at a -20 to all attacks; ok sure but you have a powerfist, only plot would cause you to use another weapon at the same time.

To make this clear: Everyone would look at this guy and think he is somewhat "special".^^