If the vacs are in series the 'side horn ' thing on one vac will be connected to the vac input of the other ... if they are both independent on a flat manifold .. its parallel
the reason i ask , i believe a booster inline will decrease system lift on a series machine ... but will not affect lift on a parallel
You have two three stage 1400 vacs .. so amusing its a parallel machine you have about 135" lift and 200cfm
a vac booster will work well on a parallel machine ... but as sean says , no one really enjoys the hassle associated .
another option is importing two new type motors the same size as yours ie 5.7
these are made by ' electro motors ' in the usa ...
mytee use them in some machines often refereed to as HP motors
cross American has hinted at using them ...
the only link i have to 230v version here , they are rebranded as ' lighthouse '
two models of interest ... a two stage and a three stage ... these are a similar fan design to the ametek 6.6
two of the two stages would give you about 130" lift and 260 cfm
two of the three stage version measures about 145" lift and 240 cfm
the two stage has quite low amp draw also .
two stage model .. 116472-AL-OD-
230 volt
three stage model ..116765-OD 230v
http://www.centralvacuummotor.com/lighthouse.htm