Coupling two 15 Amp circuit breaker for 20 Amp
Paralleling is NOT Allowed
You can't do this (except under very limited circumstances that don't apply to your situation). The problem (ignoring the code violation) is that if one of those wires breaks (disconnected at any point between device and breaker) then all of the current will flow on one wire, which is not safe as it would put all 20A on one wire. If the wire that breaks is a neutral wire then things are even worse because then the breaker will never trip because it is protecting the hot wire with the assumption that the neutral will carry the same current as the hot wires. (In a 240V-only circuit, the neutral carries the difference between the hot wires - as little as &quot;0&quot;, but in a 120V circuit it carries the same as the hot.)
You also must NOT simply replace the 15A breaker with a 20A breaker. Almost definitely the existing wire is 14 AWG, rated for 15A, and not 12 AWG (or larger) rated for 20A. So upsizing the breaker without upsizing the wire will also be a real fire hazard (and code violation).
The only real solution is to replace the wire and the breaker (i.e., a new circuit) or to find an oven that is rated to run on a 15A circuit.