To make it possible it looks like you will need an two extra lengths of vac hose and a Y connector. If you disconnect the exhaust from the first "metal" motor and direct that out of the machine using the same hose this will then free up the intake of the second "plastic" motor. You will then need to attach a piece of vacuum hose to the intake and then connect that to the the other hose which goes to the intake of the first motor using a Y connector. You can then connect the Y connector to the waste tank using another short length of vac hose. You will be able to get a Y connector from Altec.
Some things which will improve performance a lot will be replacing the plastic motor with a newer electro motor, John Kelly has these in stock and they seem to be the best price at the moment. Using all 2" hose for all the plumbing. and changing your machine inlet for a 2" inlet. If you feel really adventurous adding an extra vacuum stack to the waste tank will also improve the performance however that may be easier said than done.
I hope that all makes sense.
Plenty of room on the manifold for a third, or you could perhaps get a pre made turbo manifold And run it with an inline switch. the booster pod would be another option but the 6.6 would have lower lift then some of the electros, depends which you fitted.
I see cleansmart now offer the choice of 2 high powered electro 5.7s in place of the lamb 6.6s on the airflex storm (if i read it correctly)
I just had a look and that looks quite interesting. I think I will make my machine in to an "Electro Storm" With an Electro booster. That should give somewhere in the region of 330cfm and (11hg) 150" water lift.