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.