Relays are cheap. I'd suggest installing a dedicated relay triggered from the compressor power wire in parallel with the relay controlled by the temp switch. The electric fan operation becomes an either/or situation, controlled by engine temperature or air conditioner use. Be aware that most original-equipment electric fan switches turn the fan on at 220 degrees, so you'll need a good 15 pound or higher pressure radiator cap to allow the coolant to get hot enough to turn the fan on. Some aftermarket fan switches are adjustable, so the fan can be set to turn on at a lower temperature if desired.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!