I am going to move this thread from the FileMaker Community Feedback Space, which is specifically for input on the Community itself, to the Discussions Space where you should receive more views and potentially more advice on this topic!
Don't see how you would use a conditional value list to get this result.
What I see could be produced with two portals, each set to link to the players from a different school
You import your records from the excel files as one record for each player. If each row or record has a field that identifies the school, you need only set up a "game" table where you have two fields for entering the same school identifier to use in a pair of relationships to the players table.
Game::School1 = Players1::School
Game::School2 = Players2::School
Players1 and Players2 would be two occurrences of the same table into which you import your spreadsheet data
A portal to Players1 and a Portal to Players2 on your Game layout could then list your two rosters side by side.