3 line percussion staff can't add tam as separate rhythm?

So I have a 3 line staff with sus cym, tam and bass drum. As can be seen in the pic I can’t add the tam as a separate whole note from the bass drum. Why can’t I do this? I did search for this but didn’t find an answer.

In the Edit Percussion Kit dialog, you can change which voice the corresponding instruments belong to. It looks like that they’re assigned to the same voice at the moment.

Thanks Lillie, I didn’t even know that was a thing.