To install this package, run in Emacs:
M-x package-install RET xref-union RET
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'. You can also manually make use of `xref-union' by adding an object of the form (union XREF-BACKEND-1 XREF-BACKEND-2 ...) to `xref-backend-functions'