Have gas furnace,ac,adding outside wood furnace motor operates off a low voltage relay at stove.Adding second heat only thermostat to control stove.Want to leave gas as is but turn thermostat lower as backup.Under stand need relay in air handler to control fan from both thermoststs .How do I wire up 24v relay to control fan from both thermostats?

I'm not entirely sure why you need both thermostats to be able to actuate the fan, but if that's really what you need to do I'd suggest putting two relays in parallel - one for each thermostat.

A relay is basically just a switch that can be turned on or off with a voltage, rather than having to physically "flip" it like a light switch. There's a power source for the fan (probably 120VAC, I'm guessing), and the relay goes in series with that power source. Whenever the 24V signal is present on the control line for the relay (from the thermostat), the relay is closed (i.e. the switch is turned "on"), and power is allowed to flow to the fan motor, thus causing it to spin. If you want the fan to be able to be turned on from either of two different thermostats, you need to have two separate relays. You then connect both relays in parallel with each other from the 120VAC power source to the motor. Finally, you connect the low voltage control line from one thermostat to one relay, and the low voltage control line from the other thermostat to the second relay. That way, if either thermostat (or both) is sending the signal for the motor to turn on, at least one of the two parallel paths will allow power to reach the motor.

You should be able to find a duplicate relay to your existing one by searching online. If you can't find an exact match, you should easily be able to find an equivalent one.

All that being said, there's possibly a much cheaper and easier way to do this, but you'd DEFINITELY have to check with the experts in the heating and air conditioning forum to find out. It depends how the thermostats' output voltage is generated, and unfortunately I'm not familiar enough with them to know. IF the "off" impedance of the thermostat is high and if both thermostats are being powered from the same 24VAC transformer, then you could simply connect BOTH control lines (from the two different thermostats) to the one relay. Like I said, though, that only works if the "off" state for each thermostat presents a high impedance. Otherwise (for example, if the thermostat signals the "off" state by shorting the output to ground - i.e. a low impedance), the "off" thermostat could inadvertently short out the "on" one and damage it.

I hope this helps. I'd definitely cross-post this question in the heating and AC forum if I was you. That's where you'll find the experts for this kind of stuff specifically. Good luck!