A separate relay operating one of the two fans would solve the parallel operating problem.
Four wires of suitable grade.
Assuming the resister pack switches live (?), you will need two earths (ie same earth, two wires) and grab a live off the back of the alternator (with a suitable fuse - 30A).

If the resister pack can run the big fan fully, you could always reverse the wiring and run the old fan via a relay. I wouldn't want to have to wait for speed 3 to get the big fan to full speed though. Engine cooling takes priority over aircon, or being "nice" to the old fan!
Aux fans aren't that expensive or difficult to replace either.

The thread i neded because i tend to retire visco/disco tech and install spall fan and wire it.
Im just interested in performance of new setup?