Exactly as the title says. Only allowing this feature whilst being online seems quite pointless and defeats the game from being 'idle'.
#Allow Offline Party Combat (2+ players)
dense rose
vernal widget
Different login times would make this not work very easily
Every time you log in you restart the task you were doing, from what i can tell, so if you logged in before the other person (assuming it doesnt reset it) and left the group, it would have to account for that as well
The only way i see this working is if accounts are always treated as logged in