STAT tables for families which consist of two or more variable fonts

I've attempted writing this issue three times so I'm writing up a common scenario I encounter.

Many families hosted on Google Fonts have been drawn using two glyphs sources e.g [Nunito](https://github.com/googlefonts/nunito). One for Roman styles the other for Italic styles. Since both sources are not MM compatible, we'll end up with two variable fonts. Since Fontmake will generate from each glyphs source independently, neither font will reference the other font in their STAT table. We ideally want STAT tables which are similar to the example given in [MS STAT table spec example 5](https://docs.microsoft.com/en-us/typography/opentype/spec/stat#example-5-a-family-comprised-of-a-non-italic-variable-font-plus-an-italic-variable-font).

I understand that fonttools' varLib is only able to produce a basic STAT table at the moment. However, when it can generate better STAT tables, I feel fontmake should be aware of all the fonts that exist for the given family and produce STAT tables based on this knowledge.

I find this table rather hard to follow. The spec insists that all variable fonts need this table which is fine. However, are the tables which fontmake currently generates good enough? what are the implications if we ship families which consist of more than two fonts and their stat tables don't reference each other?

Please close this if you feel it isn't relevant Fontmake. I've read the other STAT issues and feel this warrants its own discussion.