HOW TO COVERT A SINGLE MIXER FAUCET TO A DUAL HANDLE?

i have a single lever shower handle that works as a mixer for the hot and cold water to produce the desired temp at the tap. recently i installed a tankless water heater and it constantly sets off error codes because of the single mixer handle confusing it's call for hot water (the manual states single mixers could create problems). i'm looking to convert my single mixer handle to a two lever(one hot, one cold). can this be done without removing the entire fibergl tub/shower. or is there another alternative to control the temp at the tap instead of really hot then really cold showers