I went through my RM2 books last night to see what I could dig up on Two Weapon Combo in general and I didn't find anything regarding fumble ranges, only negatives for using TWC, development costs, parrying limitations, proper use, etc. In the sessions we ran, I *believe* we used the normal fumble range for each weapon as you are making a single attack with each weapon. TWC eliminates the -20 for the off-hand attack, but imparts -25 if you choose to fight with only 1 weapon from the TWC pair (unless you've also developed use of that weapon as a non-TWC skill).
I looked in RMC-II, AL&CL, and Arms Companion (all RM2 books). Surprisingly, there was nothing in Arms Companion.