[2 UMD game] Insert the second UMD?

[LunaMoo]

In that case I would recommend opening new issue here mentioning as much information as you can about the problem, of course including game's name as it really could be anything, the feature might not work with your game due to some bug in the feature, maybe even in the game itself.

Might even be a case of game not providing clear instructions, most of those multi-umd games are japanese and sometimes game devs were making weird stuff like an anti-piracy protections requesting to unmount memstick(this can be done in ppspp from system settings) and who knows what else.
Kind of like that meme with japanese flag treated like a pie chart "What's happening in japan?" with one option where red color means "some weird shit":X.

Eventually it could be some UI problem on mac. Windows and android aside both support and feedback around other platforms tends to be pretty limited, that means not only there are little to no devs interested in helping, users don't usually report things as often either.:]