Dear Sir,

This is Piston, just join in.

Can I check with you how are you going to wire up 2 relays from 2 separate power source so that either one of the power source on will switch on the 2 fans (Two pin with voltage and ground)?

What if both power sources are on at the same time? Will there be intermittent on the fans running? How to prevent this?

Could you provide an attached circuit to illustrate the above?

Best regards,
Piston

Hummmm. How about one double throw relay. Connect the fan to common terminal. Connect the most used power source to the NCC terminal,and the other to the NOC. Power the coil when you want to switch to the second power source. If neither of the wires in the 2 power sources are a common neutral or ground, It might be best to use a double pole, double throw relay.

If you reversed the NCC and NOC terminals above, and connected the first power source to the coil, then if the first one failed, it would automatically switch to the second.

Make sure the relay is rated for the voltage and amperage of the fan.