Try giving Borcha and Marnid close-combat weapons:
--For one game, I started a Merchant character and played him up to level 20. I grabbed both Borcha and Marnid ASAP and gave them both bows. The only combat stats I advanced with them were related to bows. By the time I had reached level 20, I recorded the following stats on a piece of notebook paper:
Borcha and Marnid---
Killed me: 8
Killed friendly troops: 21
Killed enemies: 6 (most of them were lucky across-the-map shots)
Got killed: 19
--Now, I started a new game with a Merchant and tried to recreate nearly the exact situations. After grabbing Borcha\Marnid, I fought the same types of enemies, I wore the same armour, etc. and I played that character up to level 20. EXCEPT I armed Borcha with a double-lance and Marnid with a bastard sword. The only combat skills I raised with them were close-combat related. Guess what?:
Borcha and Marnid---
Killed me: 0
killed friendly troops: 0
Killed enemies: *32*
got killed: 2
I think that settles the issue...