Sounds like you need to use CB Query Field or CB Code Field. Then use a Query Select or Code Select fieldtype. Which you use depends on if you need to use PHP (CB Code Field) to output the select options or SQL (CB Query Field). This will allow you to have their query or code account for the value of the other field by substituting them in with [FIELD_NAME] and using them accordingly in your PHP or SQL.

The problem comes with updating tname when tnum changes and updating tnum when tname. That's basically an infinite loop. You can update tname based off the value of tnum using CB Core Fields Ajax and its Update On (see Integrations > CB Core Fields Ajax) functionality, but you won't be able to update bother of them like what you're wanting as they'll get stuck in a loop.