shard seance I get the one part and you get the one ability but then there is this thrown in
A medium can invite his allies to participate in his
seance—shared seance (see page 31) and [ /b ]certain spirit powers
affect other participants.[ b ]
so that means no matter what level the Medium is if they channel a Champion the others only get a +2 to damage rolls?
Simply put, yes. No matter what your level is, the seance that is shared with the party will be a fixed number (using your example, allies will only get +2 to nonmagical attacks when you channel the Champion spirit)