ABSOLUTELY NOT.

You can use a 30A circuit for electric heat, but not larger.
Even if you could use a 40A you CANNOT parallel #12 wire of it. You would need #8cu.

You need two circuits. End of story.

First, based on the question, I doubt that the person has enough experience or wiring knowledge to do the job correctly. Get a pro to do it.
Based on the information posted, a 30 amp double -pole breaker could be used to supply those heaters using #10 conductors. Both heaters would be on that circuit.

All the wire used in a circuit must be sized for the protective device. So, no, you can't connect two #12 cables to a 40a breaker. Not even if you're sure the loads are appropriate. Not code compliant, not safe.

If I'm not mistaken, a permanently installed heater is considered a sustained load, so is subject to the 80% load rule. Thus a 20a 240v circuit can supply up to 3840w worth of heaters and a 30a 240v circuit up to 5760w.

First, make sure your service has enough capacity to handle the additional load. No point in proceeding if you're just going to trip the main on some subzero winter night.

If that's up to snuff, to power your heaters, I'd probably install a 100a main lugs subpanel. Feed the sub from a 100a breaker in your main's last remaining double pole slot. Then run your heaters from breakers in the sub. It sounds like two 30a circuits (with #10 wire) would do it.

If some of this is confusing, I suggest you get a copy of Wiring Simplified and study up before proceeding. Or just call a pro, as Bus suggested.