OK, I’ll be assuming the following:
1. You received a two-track (stereo) mix.
2. You are connecting to your DigiBeta deck over SDI for both picture and sound.
3. You need to deliver this stereo mix on both 1/2 and 3/4 of the tape.
In Audio Project Settings, toggle to Direct Out (by clicking on the button that now says “stereo” or “mono”.
At the lower part of that window, choose to switch SDI audio on, choose 4 tracks, 20-bit (Digital Betacam has 4 ch, 20-bit audio).
In the sequence, place the mix on A1/2 and A3/4.
In the Audio Mixer tool, make sure that at the bottom of each slider, the right routing is chosen. A1->CH1, A2-CH2, A3->CH3, A4->CH4.
Then output.
If you were given a stereo mix, then that is what you lay to tape, IMHO. If you are in doubt about whether you need to deliver mono or stereo, consult with the folks that supplied you the mix and check the delivery specifications. There’s no telling what they are, since they vary.
If you are eventually exepected to make a mono delivery, you will need to set this up differently, and you will need to make some level alterations if you want to stay within specs. If someone else has been doing the mix, they should be the folks worrying about that, IMO.