[InactiveUser45]

I believe the magic lies somewhere in here :

By default, the emulated guest system receives the MAME hotkeys, instead of them being perceived by MAME as MAME key combinations. At least on MS-Windows, you can circumvent this behavior by hitting the 'scroll-lock' key while in and focusing on the MAME window, so the hotkeys reach MAME instead of the emulated guest. After that, you can press TAB to enter the internal MAME Interface and use the 'File Manager' option to select another CD. Just press 'scroll-lock' a second time to revert back to the original behavior.

Hope this helps.

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I have not successfully mounted multiple CHDs and succeeded in booting. For whatever reason that just has not seemed to work.
It's frustrating because the CHD I'm using (posted here, I'm pretty sure) has only ~400mb of free space which is barely enough to install anything, let alone actually copy a TAR from a CD image to then install. But I do feel like this must be possible, so I'm hoping someone has some insight.

Ah. so either: your magic incantation of 'chdman createhd' does not create a disk that Irix likes, your mame commandline using the 2 disks is incorrect, or you have uncovered a bug in the MAME emulation.

Could you share the (assumed to be correct) commandline you used to create the CHD, and the commandline to then start MAME using the 1st and 2nd disks ? Maybe this may be of interest to people.