The Chainsaw and all its assemblies aren't considered "knives" for purposes of MMB. You still get the resistances if you have a knife in your prepared slot, but you won't attack with both weapons if a Chainsaw is in either position. That being said, excluding DS from the question, I'd say the Butcher's Cleaver combined with a power-modded Chainsword would be best (and most fun) to play with, as I demonstrated in
this game. (See the part where it says "knife-juggling strategy")
Of course, the Longinus Spear or Azrael's Scythe does more damage per hit than any combination of knives; strictly speaking, combining Chainswords isn't the most practical, but it is pretty awesome. :)
EDIT: Now that I've done some calculations, it seems I spoke to soon: At lower Brute levels, LS and AS do more damage per hit, but at Brute 5 and SOB 5, dual Chainswords or the CS/BC combo overtake them. At first glance, the Ripper's double attack speed makes it do damage faster than the CS/BC combinations but then you consider the BC's Blademaster effect, which makes the attack take zero time and makes all my calculations fly out the window, since division by zero is undefined. :P
tl;dr: Chainsword/Butcher's Cleaver is the way to go, but Ripper is a close second.