I'm not a code expert, but I see nothing wrong with feeding both panel busses from a single hot leg. You would not have to replace the 30A breaker.

If you have been getting along with 30 A single hot leg, then 40A with two hot legs is as much total power as 80A from a single leg and considerable overkill.

You should be just fine with either 30A or 40A at 120V. You could also use 20A 120/240V on 12-3 which increases your total capacity, unless there is some 120V load that needs more than 20A by itself.

Fotostat,

The power to your subpanel is limited by the breaker in the feeding panel, which in this case would be a single pole 40A circuit. Feeding that wire to both legs of the subpanel wouldn't provide any additional power, but I suppose would give you additional slots for breakers. Regardless of code violations, I wouldn't do it because if someone installed a double pole breaker for use in a multiwire circuit, it would overload the neutral. Very dangerous.