I would like to know if it is possible to use multiple GlyphData.xml files at once with the same font.
What are you trying to do?
Dividing all the custom entries I make into to multiple files to make it easier to sort and find certain entries. As an example, putting the glyphs of certain scripts or sets of glyphs like combining marks into their own files.
I’ve always wondered why the glyph data was part of the app and not part of the font. What happens when there are four or five designers working on a project with glyph data and they all have their own GlyphData.xml files?
Because the GlyphData.xml is shared across all fonts. You can also have a secondary GlyphData.xml stored in folder so that it is applied to only the Glyphs file in that folder. If you work as a team, consider using such a GlyphData.xml so that it gets synced with Dropbox/iCloud/Git/whatever else you are using.
Or, use Edit → Info for Selection… inside of Glyphs to set the glyph properties directly inside of Glyphs. Then, they are not shared with any other Glyphs files and do not require an external GlyphData.xml file.
But would it be possible have different GlyphData.xml for different purposes associated with the same font and used at the same time?
That can get tricky. What if the same glyph is defined multiple times, which version is used, the one from file A or the one from file B? I don’t think this is currently possible and there would be some non-obvious decisions to make if such a multi-file setup was implemented.
It will be possible soon. But the issues Florian mentioned need to be solved by the user.
In my case, I would simply check and be sure that no list contains duplicates. Having the possibility to have multiple active lists would also make it easier to divide custom pua entries from encoded ones that have yet to be added to the main Glyphs xml file. It would thus also be quicker and easier to send the data for missing entries over to you.
I vaguely remember that Glyphs applies GlyphData.xml files it finds in the parent folders? That would allow you to have multiple GlyphData.xml files (with a somewhat unsightly folder structure). Haven’t tried it, though.