Inserting a Hyphen in between a composite primary key

I have more of a conceptual question. Is there a way to model the following:

CREATE TABLE AssetPair (
      older_asset_id INT8,
      newer_asset_id INT8,
      PRIMARY KEY (older_asset_id, '-', newer_asset_id)
  );

My issue is that older_asset_id and newer_asset_id are incrementing IDs so the following edgecases are possible:

older_asset_id: 1, newer_asset_id: 234 --> PrimaryKey:1234
// same Primary keys
older_asset_id: 12 newer_asset_id: 34 --> PrimaryKey:1234

If I was able to insert a hyphen between the two columns I'd be able to avoid this issue. Any thoughts?