Two Freepbx-Installations, shared extensions

I am using Freepbx with two trunks on two different sites.
I would like to reach the clients on the other site by several extensions.
How can I achieve this?

IAX trunk between 2 freepbx servers - General Help - FreePBX Community Forums

How to set up an IAX2 trunk in FreePBX | MangoLassi