Is it possible to make proper interpolations if a font file has both roman and italic as masters? Or does that complicate things too much? Better to keep roman and italic separate?
Technically it is possible, but it’s messy: think of kerning groups, sidebearing formulas, features and stuff like that.
Depends on your design. If the italic is just an optically corrected slant of the roman, there shouldn’t be a problem.
If you want to change shapes, such as have a single-storey a for the italic instead of double, you need to create (for instance) an a.ital glyph (the suffix is arbitrary), as you can otherwise not control kerning and spacing properly.
This means you need to add a Rename Glyphs parameter to all italic instances with a=a.ital etc.
Also add a Remove Glyphs parameter to all instances with *.ital as the value. For variable fonts, you need to add a rule at a certain breakpoint to substitute these glyphs.
So, in summary, if you don’t have changing shapes, it’s not a problem to keep the same file. Otherwise, it’s a big mess.