One family - two variable fonts: Regular and Italic in two separate files

Problem:

I have made two variable fonts - regular and italic. Both have four masters and nine instances. I want both fonts as one family. When I put just one in InDesign font folder each one works perfectly, when they are together the problem starts. What should I do to make them work together in InDesign/Illustrator/etc.?

Solution:

Let's take two VFC files Sample-VF.vfc and Sample Italic-VF.vfc. Each includes 3 weight masters and each must include the Italic axis:

Masters in Sample-VF.vfc:

Masters in Sample Italic-VF.vfc:

Export two var fonts from FontLab using File > Export Font As...:

Sample-VF.ttf and Sample Italic-VF.ttf

Prepare two yml files that will be used to compile new STAT tables: sampleroman-stat.yml and sampleitalic-stat.yml. You can use any text editor:

Now open the Terminal window.

Run the following command to install Google Font Tools:

pip install gftools

After 'gftools' is installed successfully run the following command for your upright font:

gftools gen-stat [location of Sample-VF.ttf] --src [location of sampleroman-stat.yml]

and the same for your italic font:

gftools gen-stat [location of Sample Italic-VF.ttf] --src [location of sampleitalic-stat.yml]

You should get two new files with fixed STAT table that will work together in Adobe apps. Here is how this looks in the Terminal window:

Sample files are attached.