Can two people edit a tour at the same time?

Yes, for hotspots. Two people can edit the hotspots of one panorama on the Hotspots tab at the same time and both keep their work. The tab saves only what each person changed, hotspot by hotspot and field by field, half a second after the last change, and adds it to what is stored. Different hotspots, or different fields of one hotspot, both land. Only when you both set the same field of the same hotspot to different values, or one of you deletes a hotspot the other changed, does a box ask you to choose Keep mine or Take theirs.

Two things are not live. You do not see the other person's changes as they make them: they arrive with your next save, and a reload shows them now. Undo takes back only your own changes and leaves a hotspot someone else has changed since.

This works on the Hotspots tab; on the other tabs, take turns. To add the second person, the tour's owner shares the tour on the Settings tab under Project Collaborators; an Editor or Admin can edit hotspots, a Viewer cannot. Edit hotspots with someone else at the same time has the details, and Invite collaborators to a tour and set their role the roles.