Can I use existing heater wiring to supply a sub-panel for two circuits?
If the owners manual/installation instructions say two 240v 15A circuits are required then you are good to go if you mount a new subpanel with two 240v 15A breakers. Those instructions are part of the NRTL Listing and override general NEC provisions.
If the instructions don't specify then IR saunas are normally considered &quot;fixed electric space heating&quot; which NEC 424.3 designates as continuous load, so circuits are calculated at 125% of nameplate. So the branch circuit from a new subpanel would need to be 20A.
For the feeder calculation there is a little, but very little, wiggle room, for the feeder for the new subpanel. Then general rule in 220.51 says the feeder is to be calculated at 100%, but then there is an exception that allows an authority having jurisdiction to allow reduction under cycling conditions. I doubt many local inspector would grant this permission.
Edit: Now seeing the nameplate says 10.4A, not 13A. Then 10.4 x 125% = 13A. It appears the 13A is the circuit requirement, not the actual heater draw. So two 15A receptacles on separate 15A breakers being fed by a 30 feeder should be fine.