I have a garage heater but I don't want it to run at all if the temperature outside is above 45 degrees
Your idea of having two thermostats will probably work, but the thermostat you are searching for might need a little help.  There are two ideas I have.  First, a heat pump system needs a thermostat outside so the compressor doesn't run when it's too cold outside.  Thermostats such as the Goodman OT18-60A perform that function.  Now, that is designed to cut off the power when the temperature is too low, but that in conjunction with a "normally on" relay might work for you.

The other thing you could search for is thermostats for pool or spa equipment.  Pool freeze protection relies on an outdoor thermostat, so you might find something there.