Which thinset to connect USG Durock® Brand Cement Board with EdgeGuard?
The cement board instructions are for a wet area.   Since there is no need to waterproof behind a toilet you should not be worrying about this.
I would buy cement board tape and stick it across cover both boards.   As you trowel the mortar for your tiles feel free to push as much into the mesh that it can hold.   That is your install.
If you try to set your seams first and you let them dry inevitably you will have a ridge.   That ridge can cause a lot of depth issues when applying mortar to the wall and tile (backbuttering).   I advise not cover your seams in non-wet areas and apply the mortar in one go.