So I must have misunderstood the CFX plumbing, I guess they have a "single" exhaust? This leads me to believe it's actually a 100 cfm machine with the vacs inline, one sucking behind the other "series".
So Rotovac told you to hook your dual vac unit to this dual vac unit, giving you 4 two stage vacs in series...still 100 cfm with a pile of water lift?
If you need an auto pump out that bad, then g'head. But honestly, at some point I would just sell teh old unit and put $2500 in to a high performance self contained 15 gal M5 with twice the vacuum and twice the pump and be done with it.