I just want to clarify what you're asking.
You have a sub-panel in your garage. How large is the breaker feeding this sub-panel? (located in your house)
This sub-panel in the garage, has two 15a breakers, one of which is connected to the red, one to the black wire in a run of 10-3 wire.
What does this circuit power? I'd assume your 240v heater? Is there anything else connected to this circuit?
Since it's 10 gauge wire (as I understand from your post), it can be used with a 30a breaker, but it will need to be a 240v 30a breaker, there can't be any branches off this circuit with smaller wire, and only if your heater is rated for 30a.
Now, do you need 30a? What is the requirement for your heater? There's nothing wrong with oversized wiring, and if you're not tripping any breakers, it might not need to be changed. YOu WILL need to make sure both 15a breakers are tied together, but that's it.