Our group was behind a 5 foot tall (1.5 meter) barrier. The enemy (an assortment of Dark Eldar) were attacking our position. We had some advance notice of the impending assault and needed to defend the position. I had placed my Frag grenades and Fire bombs strategically around my defensive postion to easily reach them. I also had my utility mechadendrite equipped with a las cutter (I had used it just prior to the encounter) . Several DE rushed our position and I put down my hand cannon and grabbed two Fire bombs. As the DE were on the opposite side of the barrier, I said that I wanted to take the las cutter that was still in my mechadendrite and light the fuse on each Fire bomb and toss them over the barrier.

The GM said sure, but I would take a -20 to hit for each. I thought a - 20 for my off hand was a given, but the additional -20 to my main hand seemed wrong to me. The GM said, it's in the book and you can look it up later. I missed with both (not terribly, and no negative effects other than me being down 2 Fire bombs and the loss of my action). Sadly, I would have hit with my main hand if I hadn't had the -20.

Looking through the rulebook now, I cannot see where it states that my main hand attempt would be at the -20.

Other than GM prerogative, can someone point out why I would take the -20 to hit for my main hand in this scenario?

Thanks.

From the errata:

If you have the Two Weapon Wielder talent you can hold two
weapons, one in each hand, and can make a single attack with
each. This is a Full Action, and both attack rolls suffer a –20
penalty (your off-hand weapon does not suffer the normal –20
off-hand penalty on top of this).

Technically, you shouldn't have been able to pull that off at all unless you've got the Two Weapon Wielder talent. Sounds like your GM was being pretty lenient.

The Boy Named Crow said:

It is still fair game if you don't have the TWW talent but you then have to factor in the off-hand penalty.

Khouri's right. I missed that part.