When two people interrupt each other, talk at the same time, their voices overlap, etc., you should transcribe what each person says on separate lines.
Example 1:
[00:00:00] Hi, what's your name?
[00:00:00] My name's...
If you don't understand what one of the speakers is saying, add the tag [INAUDIBLE hh:mm:ss] where appropriate.
Example 2:
[00:00:00] Hi, what's your name?
[00:00:01] [INAUDIBLE 00:00:02] name's...
Example 3:
[00:00:00] Hi, what's your [INAUDIBLE 00:00:02]?
[00:00:02] My name's...