Hi,
Please check the tools listed in below link and consult experts in the related forum for your question of creating mixed fonts.
Font tools list
- Visual OpenType Layout Tool (VOLT) VOLT is a tool for adding OpenType layout tables to fonts.
- Microsoft Visual TrueType (VTT) A professional-level tool for graphically instructing TrueType and OpenType TM fonts.
- OpenType font signing tool A tool developed for use at Microsoft for signing font files.
https://learn.microsoft.com/en-us/typography/tools/
https://forum.fontlab.com/index.php
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Hope this helps and please help to accept as Answer if the response is useful.
Thanks,
Jenny