There's a 40A circuit in the greenhouse that's separated into 5 individual circuits consisting of two 15 amp breakers and four 20 amp breakers, one of which is unused. If I run 2 small electrical heaters on one side, the 20 Amp circuit trips. Obviously, the two heaters are drawing more than 20 amps. My question is: If I connect the two 20 amp breakers together and try to run both heaters am I looking for trouble - will it work?
You are, looking for trouble & you'll get it. You must run each heater on a separate circuit, they will not run on the same circuit. A proper 20amp circuit will only handle 2000-watts or 17amps safely & continuously.
Look at the bottom or back of the heaters for how many Amps they use or require. You'll need an Electrician to re-wire the heater area if they are where you would like or need them to be.
Connecting 2 breakers will allow for 220v at 20 amps. What are the heaters rated for? If they are 120V connecting the breakers will not help. If 240 is needed and less than 20 Amps you will probably need to change the wiring. To legally connect the handles a listed handle tie is required.