No, you cannot put one in series to correct a flow issue. If would still be
restricting the flow as the water still needs to flow thru tankless , thus
still the 5GPM, you can't get more out of a tank than you are putting in to
it. Parallel devices add, series devices is the flow of the lowest device.
Putting 2 in parallel would give you a 10GPM flow at the shower, 3 15GPM, 4
20GPM etc

Takagi has a controller to allow you to so this with their T-M1s:

http://www.tanklesswaterheatersdirec...agitm1main.asp

Michael Thomas
Paragon Property Services Home Inspections
http://www.paragoninspects.com