Doulas just set their limit as to how many births a month they want to take. I know some who will do 1 or 2 and some will do 6-8 but there are fewer of them.

I quite happily will have two clients on one month due on the same day, because you could have someone due first week, someone due last week and they both come in the middle anyway! One two weeks early, one two weeks late. I even had two clients being induced on the same day but I was okay with it, because one was having gels and I knew it would be at least 24 hours before she went into labour. The other was a third time mum and she bet her induction and was a day early anyway... so worked out perfect

You can never plan it but I have never had two births at the same time since 2005 - back to back but not at the same time. I think someone knows something