There was thread about this the other week and it wasn't it wasn't a simple answer.
This is how i would do it, but you may get a different answer off someone else. I've done a number of these and been very confident with the results, bearing in mind there are some clever people on here and if they come up with a better solution, i'll use it.
Firstly which thermal boards are they??? If they are the rubbishy foam/polystyrene type one that crumble, then i really woludn't use them, they compress after time, use a
cement board like marmox, much better.
Obviously the wooden musn't flex or bounce, if it does then again insist it's sorted.
Ideally you would want a new plywood base to
tile on the for the wooden floor, another thing to think about.
I prime both substrates with SBR then using a 10mm notched trowel i trowel a flexible
standard (or fast set) adhesive onto the floor. I've been using Mapei Keraflex Maxi lately and it's bloomin lovely, and boy it goes hard!!!
Then press the boards into this. Think about the
tile size and how the boards will be placed, some tiles are 600mm as are the boards, don't set out the tiles in the same way as the boards!
After pressing these down, screw the boards down onto the wooden area using the plastic collars recommended, making sure you only drill into the boards and not the ply, i think someone mentioned 15 screws per Marmox board onto wood.
Do all the area and leave to dry.
After the underfloor heating has gone down and levelled in some cases especially stone then i'd use Ditra matting onto of the levelling compound and then
tile onto this.