I tried merging 2 fonts in the font editor FontForge.
A curious thing happened. The original glyphs in my font became tiny while the imported glyphs looked the right size.
When I tried to export, to see what the font would look like for real, a message told me that the em length should be a power of two but was 1000; it let me know that I could change this is in Elements>Font Info>General, so I changed it to 1024. Then, while FontForge was doing its stuff it crashed.