Version 0.2.0 of package Xref-Union has just been released in GNU ELPA.
You can now find it in M-x list-packages RET.

Xref-Union describes itself as:

  ==============================
  Combine multiple Xref backends
  ==============================

More at https://elpa.gnu.org/packages/xref-union.html

## Summary:

  This package provides a way to combine multiple Xref source
  (e.g. Etags and Eglot) and have the results all at once.

  To enable, toggle the `xref-union-mode' minor mode.  If you want to
  exclude certain modes, take a look at the user option
  `xref-union-excluded-backends'.

## Recent NEWS:

[Not provided 🙁]