Is your feature request related to a problem? Please describe.
If a key on a touch layout needs to output two Unicode characters, the designer must assign a T_xxxx label, then use the .kmn file to cause that T_xxxx code to output the two Unicode characters.

Describe the solution you'd like
Just as using U_zzzz on a touch layout produces U+zzzz, using U_xxxx_yyyy would produce U+xxxx and U+yyyy.

Describe alternatives you've considered
The current use of T_ codes and code in the .kmn file.

Additional context
Would need to consider how to handle non-BMP characters.
Once you allow two characters, someone is sure to want three, etc.