Here is my suggestions on what to do for an XI Roll.
1) An XI Roll immediately becomes a spell-type buff, and therefore will not take up 1 of the 2 positions for a Roll-Type buff.
2) The above would then open up the ability to have 3 or 4 COR buffs on the party and yourself. The limitation could be 2 XI Buffs and 2 Roll Buffs, regardless if the 2 bonus rolls were XI.
3) For the XI Rolls that become spell-type buffs, double the duration.
Making an XI roll become a spell-type buff, there could be an ability similar to Double Up involved. Let's call it "Butter" (because you're on a roll. Get it? >.>), at least for this explanation. So you Double-Up a few times, hit the XI, then hit the Butter command and that roll becomes the new spell-type double-timed buff. This command will allow the COR to have greater control over what gets buffed.