One circuit on two 20 amp breakers
NO.

First -- the job of the breaker is to protect the wire from overheating and BBQing your house.  A 40A circuit would require 8AWG copper wire, not the 12AWG that is currently in your walls.

Second, you are not the circuit breaker factory, which means you have no idea how such a combination would perform, and are forbidden from doing this by NEC 240.8:


  240.8 Fuses or Circuit Breakers in Parallel. Fuses and circuit breakers shall be permitted to be connected in parallel where they are factory assembled in parallel and listed as a unit. Individual fuses, circuit breakers, or combinations thereof shall not otherwise be connected in parallel.