I'm no expert on this stuff, so take my opinions (cause that is all they are) with a grain of salt, I could be very wrong.
I have done a lot of reading on the subject and I've built my own cyclone from scratch, for whatever that is worth.
I do not think you will see any significant increase in performance.
Static pressure counts for next to nothing in a DC, Cubic Feet per Minute, CFM and Feet Per Minute FPM to show the speed of the air. In a vacuum system, like a shot vac, the static pressure is important, but in a DC system, the CFM is king.
I am fairly sure that hooking up two smaller DCs does not really give you any advantage. What scientific method have I used to come to this conclusion? None, that is not my area of expertise, but, I think if this did work then why is everyone not doing it? You can often pick up cheap 1 HP units on sale, so why don't guys buy three or four and hook them all up?
Honestly, if this worked, don't you think all the DIYers out there would have done it, and shown it worked and be using this cheap and simple cheat?
The fact that now one does this, well that weighs very heavy in my thinking that it would not work.
The more I think of this, the more I think it is a bad idea.
Blowers work by compressing the air, moving it faster, and to do this you need power, the more air moved the more amperage drawn, the better , more powerful motor you need.
Do this simple test, if you have an amp meter around hook it up to you DC's motor, now remove the hose from the intake and block it off, and remove the hose from the filter side and block that off too. Now no air gets into the blower, you will find that the amperage drawn is low, as there is no work to do, the motor and the impeller just putt along. Next remove the blockage and try the test again, with no restriction on the intake or the filter side.......
but try it only for a second or two and at your own risk, with the motor running the impeller in the blower with no restriction
With all the air it can churn, the amperage reading will go way, WAY up, as the motor works harder having a lot of air to move......
you can quickly burn out a motor doing this so you have been warned, be careful, don't come crying to me because you let the smoke out of your motor
Now if you put two blowers in series, as suggested above to the first blower is in effect super charging the air stream for the second blower, and you will almost certainly burn the motor out in the second cheap blower set up. I would say it would work, but you would need a really powerful motor to handle the super charged stream of air from the first motor, and this would end up costing you more money than just simply buying the 5hp Leeson motor and 14" impeller you need to make a blower that moves enough CFM to catch all the fine deadly dust.
Hooking them up in parallel, as mentioned above, would not give you an increase in CFM, like hooking up two batteries in parallel, your voltage does not increase.
If you have two DCs you would be better off splitting up your work between them, maybe making one that is dedicated to only your planer and jointer or some such set up, that might work better for you.
I hope this helps, it is only my thoughts and opinions, I have only that to fall back on.