Thank you, Ryan_IK!
I could make them work by assigning iRig Keys I/O to "Receive From" of both MMC and iRig Keys I/O. Assigning iRig Keys I/O to the input of both devices shows a warning, so I thought that this setting was not good
BTW, I can stop playing/recording by left button though, can I assign "Return to Start on Stop" to the left button?
If this is not a suitable topic to this forum, I will ask somewhere.