Congrats on getting the 2 Fathoms.. I think that was definitely the right way to go.
I have much of the same equipment you do (ex: C2600, MX151, 2x F113's, MEN220); having said this, I have not actually tried what I am about to recommend so take this as perhaps an experimental starting point.
And just to level set.. I assume this setup is in a theater (due to MX122 and mention of rears) but uses HT bypass to the C2500 and MEN220.. If this is the case, do you want to also use the subs in both theater (MX122) and 2 channel (C2500 & MEN220)?
I guess I cant really reveal my "complete" suggestion until I know the answers to the above, but in essence, I would advocate a starting point of summing the entire signal to both subs (via balanced splitter) and using the room correction on the F113's vs. going stereo and using the MEN220 to match the subs to the main speakers.