Using two 12/2 cables for 30 amp?
1) With only a couple exceptions (of which yours is not one of them) the National Electrical Code does not allow the paralleling of conductors less than 1/0 AWG in size. The reasons for just about everything in the Code is safety.

2) In your scenario you will have neutral current flowing on a bare equipment ground wire. Another code violation because it is very unsafe.

3) Current flowing in one direction in a cable and not in the other direction can cause electromagnetic inductive heating if passing through ferrous metal components like cable clamps, locknuts, panel knockouts, etc. (The principle used by induction cooktops) This leads to melting insulation, FIRE  and everybody dies. Not a pretty scenario. Again this is prohibited by the code for the, hopefully, obvious reasons above.

So, short answer to your unsafe proposal is NO don't do it

Run a properly sized cable. It is not worth your life or your family's lives to save a few bucks.

Good luck!