One thermostat with a switch to control 2 heating systems
You just need a multi pole switch. If you are using a T-stat that uses batteries you may only be using two wires to actuate the circuit. 

In that case get a double pole double throw switch. (If you are using three wires you will need a triple pole double throw.)

Like this one.

Connect your thermostat to the center terminals and the boiler and the wood burner to each of the other pairs of terminals. Then switch back and forth to your heart's content.

Good luck!