If these are truly 900 watts each, running on 240 volts, then yes, you can run a 12/2 cable to a junction box and feed both thermostats.

For heater loads, the maximum wattage for a 20 amp circuit at 240 volts is 3840.

Yes, the wattage is well below maximum. I just need to know if it was ok to split the line before the thermostats. Thanks

Yep, you can split the line in any place that's legal to make such a junction. This will be an accessible box (perhaps one at the heater, perhaps at the thermostat, or perhaps somewhere else). In some cases you might be able to do so inside the enclosure of one of the heaters.