I have 2 30 amp breakers on a circuit that only has a 30 amp receptacle for my dryer. Can I tie into this circuit to hard wire a 30 amp 7500 watt garage heater?
Your heater is 7500W, which is 31.25 amps, and because it's a heater, it must be derated to 125% or 9375W or 38.8 amps.
That is too much for a 30A wire and breaker, but is "just right" for 40A wire and breaker. That is designed that way on purpose.
So you need to use #8 cable, e.g. 8/2 Romex. And a 40A breaker. The 30A/10AWG wires are not suitable for this heater.
If you want a heater able to exploit the #10 cable and 30A breaker, pick one whose output is 5300-5500 watts (22-23 amps). That, multiplied by 125%, will fall just under 30A.
If you are connecting it via cord-and-plug, Code requires you use a 50A receptacle. You can either use a NEMA 6-50, which does not include neutral, or a NEMA 14-50 receptacle (but then you need to run 8/3 cable).
If you also want to plug in actual 50A loads, such as an RV or electric vehicle charger, then run 6/3 cable to a NEMA 14-50 and use a 50A breaker.