All weapons, including bows have the same penalty structure.
Right hand MM penalties:
0x: -30
1x: -22
2x: -15
But there is an issue with handedness. All weapons, regardless of which hand is doing the holding, have a -30 MM penalty with 0x training. However, when held in the left hand only half the training is being applied. Holding a bow in the left hand with 2x training will have the 1x training penalty of -22 rather than the 2x penalty of -15.
I'm hoping this is a bug. If not, then archers, who want to utilize UCS attacks, will need to change the default from left to right hand when carrying a bow.
Off-hand MM penalties:
0x: -30
1x: -26
2x: -22
Edited to add: I tested this with a character with both 0x and 2x brawling ranks per level. The off-hand results remained unchanged.
Mark