First of all, keep in mind that there are two quests: [SOLO] and [SOLO+]. Both of you need to accept the [SOLO+] mission from the terminal, otherwise you can't enter it together. If you already have the [SOLO] mission in your log, you need to abandon it before you can pick up the [SOLO+] mission.
That being said, I also had this bug where we both had the [SOLO+] mission but I still couldn't see my partner. We fixed that by both exiting Eternal Championship and resetting both the quest (in the quest window) and the phase (right-click on your portrait, reset active phases).
Please let us know whether this worked for you!
