How many heating watts can I draw from a pair of tied 15A breakers?
Resistive heating loads are required to be calculated as continuous loads so the circuit can only be loaded to 80% of its rating.

Therefore a 15 amp circuit at 120 volts is good for 12 amps which yields 1440 watts of power. More likely the heaters are 240 volts which yields 2880 watts per circuit for a totoal of 5760 watts for both circuits. (240 volt circuits will require a 2 pole breaker for each circuit occupying 2 spaces in your panel for each one.)

However, you would have to put two on one circuit and one on the other so you have to make sure the circuit with two heaters does not exceed 2880 watts at 240 volts and then put the largest one on a circuit by itself.

Here is part of the pertinant article from the 2017 National Electrical Code:


  424.3 Branch Circuits.
  
  (A) Branch-Circuit Requirements. Individual branch circuits shall be permitted to supply any volt-ampere or wattage rating of fixed electric space-heating equipment for which they are rated.
  Branch circuits supplying two or more outlets for fixed electric space-heating equipment shall be rated not over 30 amperes. In other than a dwelling unit,  fixed infrared heating equipment shall be permitted to be supplied from branch circuits rated not over 50 amperes.
  
  (B) Branch-Circuit Sizing. Fixed electric space-heating equipment and motors shall be considered continuous load.


Good luck with your project!