What is the FLA on the motors?
What is your amp draw (using a clamp on metre) with Max open port and exhaust? (Blowers benchtest no filters)

What is the intake and exhaust diameter on each blower?

If you run them in series, they will just be helping each other, keeping their amps lowered.

No matter where you put them in parallel, with the system closed(all gates closed), they will be pulling on each other.

The dust deputy XL has a 6 inch in and out. Most 2 hp blower’s have a 4-6 inch in and out. Most 2 hp DC start losing efficiency at 20 ish feet. So, you might be able to get a 6-8 inch main trunk that runs 20 feet with 10 foot drops of flex pipe. You might be able to run two machines with 6 inch ports on a 8 inch main trunk with out over Amping the motors.

It is really a lot of bench testing. For sure you will get more CFM (with two blowers) but the dust deputy may be choking you with the 6 inch intake/exhaust. I actually had to choke my system to keep my FLA under the motors FLA.