how to measure vacuum suction

[Joe K1]

I'm attempting to repair a commercial/industrial vacuum. The contractor uses these while grinding concrete.

I've been asked to separate the load onto two 120v cords which is basically the way the 240v is wired.(so he can plug into different circuits on a residential site).

I am retired and have always been employed in large manufacturing corporations.  I have never claimed to be an expert but I have been deposed and in court over product liability lawsuits and serious employee injuries as the result of use of purchased industrial equipment.

As I read this, from my prospective here is something to consider.  A contractor, most likely with employees, wants you to modify a commercial/industrial product.  If there is an accident or injury after the modification, where does that place you, the center of attention in a room full of lawyers.  The equipment manufacturer and the contractor should have insurance so it will most likely end up in court in an effort to minimize their liability.

Potential problems that I see:
Multiple power sources to a portable machine through individual cords and circuits.  Doubtful that any governing body would approve. 
In residential use, most likely to require using circuits in different rooms.
Significant chance to sometimes have 240V line to line in the machine.
Complete isolating one power cord from the other power cord under all possible conditions.

[MstrCarpenter]

Joe, thanks for your input. I'll respond to your points, but you've already changed my direction on this.
.....where does that place you, the center of attention in a room full of lawyers. 
Hopefully we'll never need to go there.

Potential problems that I see:
Multiple power sources to a portable machine through individual cords and circuits.  Doubtful that any governing body would approve. 
I agree. I would much rather wire it 240v (they are identical except the load is split inside the machine using two "hots" instead of jumpers at the three switches) and supply the contractor with a 20a 240v twist-loc to two 120v 15a adapter. (like those offered for generators except the genders are reversed)

In residential use, most likely to require using circuits in different rooms.
They usually use their portable panels which have all the circuits and power they need; 60a single phase and 75a 3 ph. they can use one of the 240v 20a twist-loc receptacles. The "in house Electrician" is usually provided with a 6' whip w/appropriate female cap and asked to wire it to their panel. 

Significant chance to sometimes have 240V line to line in the machine.
They would share a common neutral, but there are four motors. I would put two on each hot; exactly like the 240v vacuum.
Complete isolating one power cord from the other power cord under all possible conditions.
Due to your enlightenment, I will  re-wire it with 12/4 SJO with a 240v 20a twist-loc. I'll also only supply the knowledge of how it's possible to use two 120v circuts. Thanks again for your concern and enlightenment.

[Joe K1]

In residential use, most likely to require using circuits in different rooms.

They usually use their portable panels which have all the circuits and power they need; 60a single phase and 75a 3 ph. they can use one of the 240v 20a twist-loc receptacles. The "in house Electrician" is usually provided with a 6' whip w/appropriate female cap and asked to wire it to their panel. 

I was picturing your proposal as 2 separate power cords and the operator would plug into a wall receptacle in one room and the other cord in another room in hope of being powered by separate breakers to eliminate single breaker overload.  Your explanation of the use of a portable panel an electrician making the connection to the residence service panel makes sense.  I didn't mean to kill any further discussions on the vacuum suction.