I have 2 - 15amp tandem breakers - both are 1 pole. I know that normally you would use a 2 pole breaker to create a 240v feed to the heater. However, if I connect my 12-2 wire to the 2 different breakers, I get 240v and my heater works correctly. Is there any problem to doing this rather than using a double pole breaker?

Thanks

This is very dangerous for people who perform service for the heater in the future, you have to make sure they turn off both breakers before any service work, if they get injure or kill, you may be held responsible for it, you should replace it with a double pole breaker.

Also with a 2 pole breaker the 2 poles are tied together called a Common Trip, if one side shorts out, the other pole trips also,without it the circuit/pole is still alive.

Spend the 8 bucks and do it correctly and legally.