CNC'ing both quill and knee

Have a Series II CNC that I am converting. Not my first as I have
already done a Series 1 machine.
I am looking to do a 4 axis conversion on this machine. So far I have
never needed more than the 5" travel of the quill. So the quill will
have the stepper replaced with a servo and be my primary Z axis. I
would also like to use the knee like a 2nd Z axis to give me more Z
travel. The knee should work nice as it is air assisted and has large
box ways. So....is it possible to make the quill and knee work in
concert with each other? Should I just make the knee another axis,
say the B axis, and worry about how it will work with the software
(mach 3) later?

Thank You,
Scott