The best solution that you can get for this purpose is the RME ADI-2 Pro FS.
http://www.rme-audio.de/products/adi_2-pro.php
Besides being able to connect 2 phones it gives you also best D/A conversion quality for your monitores.
In this blog article I describe the typical integration into each setup and how easy it is to switch between phones and monitors by making use of the re-map key feature: https://www.tonstudio-forum.de/blog/ind … our-Setup/
In particular, you could i.e. connect the ADI-2 Pro FS digitally via your ADAT or coaxial SPDIF output and have 2 reference grade phones outputs where you can adjust the volume for each of the phone individually. You can change between the 2 phones outputs simply by pushing the big volume knob.
You could also think about upgrading to an UFX II with 2 phones outputs as an alternative if you would like to get Durec along with this and more I/O ports.
Otherwise look for other solutions here: https://www.thomann.de/gb/headphone_pre … ImVuIn0%3D
I can't give you a recommendation here. Look carefully whether you can change volume for each phones output separately and read the buyers comments. Some products seem to have a sensitivity to hum, etc.
BR Ramses - UFX III, 12Mic, XTC, ADI-2 Pro FS R BE, RayDAT, X10SRi-F, E5-1680v4, Win10Pro22H2, Cub13