Two separate gauge wired switch in one double gang box
They can be in the same box but the breaker that feeds the switch and light with the 14/2 has to be a 15 Amp breaker. No exceptions. you might want to leave a note in the panel as to why the 12/2 is breakered at 15 AMPS. If that circuit needs to be 20 amps, you'll have to replace the 14/2 with 12/2.