If you set multimouse back to 0, do BOTH mice work in Mame?
Ie when multimouse is 0 all attached mice should do the same thing in mame.
When it's 1, Mame can "see" each mouse independently, so you'll need to update your control configuration as applicable.
And finally, if you disconnect and reconnect devices often, the mice can "change position" on mame, invalidating your config. You can check out the link in my sig for my "controllerRemap" utility for more on that.