After getting some sleep I realized the most obvious answer to the problem would be to have each Trade belong to two Nations.

package Game::DB::Schema:Trades;

__PACKAGE__->belongs_to(
  creator => 'Game::DB::Schema::Nation', {
   'foreign.id' => 'self.id1',
  },
);

__PACKAGE__->belongs_to(
  acceptor => 'Game::DB::Schema::Nation', {
   'foreign.id' => 'self.id2',
  },
);
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At the top of DBIx::Class::Relationship in the Synopsis there is even an example of this showing a many-to-many relationship which you could probably even implement if you wanted to.