What is professional practice in a program where there are two talents being recording standing side by side: mix both to center, or have left talent on left channel and right talent on the right channel?
Mastering is more about final EQ, compression, limiting and level adjustments. You appear to be asking about panning.
I don't think there's anything carved in stone about panning preferences. I try to avoid severe, full left and full right panning unless there's a good reason to do so.
I tend to be conservative in situations like this and keep the speaker's voices more to center, if not "mono" or with both tracks panned to the center because, sometimes, somewhere downstream, someone will incorrectly decide to use only one channel of my two channel audio.
Ty is of course right on with his part of the answer if you are talking about how to deliver the final finished video and audio.
If what you are asking about is how to RECORD the tracks, then the answer can be slightly different. It is normal practice for an audio person when recording a scene like you talk about to pan one person left and the other right. That keeps their individual tracks isolated as much as possible from one another so you can raise or lower the level or add EQ to one person without affecting the other one (as much). If they are really side-by-side then the bleed from the other person's voice makes that more difficult of course. Even in this situation, you would RECORD with one voice panned left and the other right, but for the final video, both voices should be panned center, unless you are doing some sort of trick 5.1 surround sound delivery. Even then, dialog is usually in the center.