I've noticed two issues concerning power-manager in an environment with multiple user X sessions: 1) Dimming monitor brightness happens regardless of which X session is active. Even worse, to 'undimm' one has to switch to the triggering X session and move the pointer. 2) When going to suspend/hibernate only the active X session will be locked.
1) Not sure what you mean with "dimming". Screen brightness? The screensaver that blanks the screen? Also, do you have both X sessions open next to each other or why are you even noticing that it dims? 2) Not sure there's much xfpm can do about this tbh. Maybe systemd handles that in a better way.
2) I agree, once systemd/logind is in place, such things can be handled much easier. 1) IIRC this was an issue with the xfpm feature to reduce screen brightness after a certain time. E.g. user2 has it set to reduce the brightness after 10s and user1 is also logged in and has the active session, than the screen was dimmed after 10s for user1! However the screen dimming was broken and fixed in the meantime and I can't reproduce it anymore. So I consider it as resolved.