Can I run two 72" 7.2amp 240V baseboard heating units on one 12-3 wire supply
12Ga wire is rated for 20 amps, so derating for continuous load nets you 16 amps, [16Amps times 125% = 20 amps] and 14.4 amps is less than 16 amps, so one 12Ga wire should be adequate for two heaters.

Whether the thermostat is rated for that load will depend on the thermostat, but from a wiring point of view 12Ga wire is adequate.

Pending your response to comment, I'd expect a resistive heater that draws 7.2 amps on 208 volts to draw ~8.3 amps on 240V, at which point one 12Ga wire won't run two (but each could be run on a dedicated 14Ga wire, which is good for 12A continuous load.)

From comment to the question: Examining the 240/208V 72" heater, Part #: 09956


Watts at 240V: 1500  - Amps at 240V: 6.3
Watts at 208V: 1125  - Amps at 208V: 5.4


As expected, being a resistive device, when the Voltage goes up, the Current goes up, and the Power (which is also heat delivered) goes up. Running a 208V (only) heater on 240 V wil also result in more current, and more power, but will probably also cause the heater to overheat, as it is not designed to deliver that much power/heat.