Yes, I understand you. However, this is tested and confirmed by me:

On the unit: Two parts of the same FID map on the unit where one is named gmapsupp along with another part named any of gmapoem, gmapprom or gmapprom1 works fine. Also, parts with the same FID named gmapprom and gmapprom1 will display, or 2 parts named gmapprom and gmapoem for example. If all the names are used for different FID maps there is no problem using all the names at once.

On the SD: If gmapsupp is on the SD the corresponding FID part on the unit can be named any of the 4 names usable on the unit. i.e. not just gmapsupp but also either gmapprom, gmapprom1 or gmapoem. While the requirement may well be that there can only be one other FID part on the unit it's naming is not limited to gmapsupp [I didn't test with two parts on the unit in combination with gmapsupp on the SD]. I know you haven't been able to get MX working except as gmapsupp and i can't explain why that is.

Here in these screenshots i've got Mainland AU on the SD card, and Tasmania named gmapprom on the unit. It also displays the same Tassie tile re-named as gmapprom1, gmapoem and gmapsupp on the unit:

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Also fully tested and working: Mainland on the unit as gmapsupp and Tassie as any of the other names or vice versa. As said above i didn't need to use gmapsupp at all on the unit, i can just use any 2 of the other names instead. All names tested and working in all possible combinations both with SD/unit and on unit only.
BTW, that 'anyname'.img is not limited to modern outdoor devices it's applicable to modern nuvis as well.