The latter option is very cumbersome since the process would be something like loading content fast boot disabled, going into disc control, ejecting the disc, and restarting the content (currently broken)

If the user wants to handle the situation I mentioned with Ratchet & Clank: Going Commando without using an external program, they'd have to make sure they used the shared second memory card or else they'd have to rely on either in-game saving to switch slot or the built-in memory card tool of the PS2 bios to transfer things over.

Closer to how other libretro cores handle save games.

The typical user won't have to worry about memory card management as it's managed for them.

I see a couple of options here, with pros and cons for each: there's a vendor in Ratchet & Clank: Going Commando that gives you free weapons from Ratchet & Clank if you have a Ratchet & Clank save file. The bigger concern would be games that read your memory card for save data from other games, e.g. m3u playlists, so you'd use the same memory card for both discs. That's not a huge concern, though, since the core already supports.

Devil May Cry 2 and Star Ocean: Till the End of Time. Yes, there are a handful of multi-disc games for PS2, e.g. I think at the beginning I will do also an option to disable per-game saves, because I don't know how behave multi-disc game (but there are any for PS2?)

Ps2 doesn't read memory card in game