Different centers and individual instructors will all have their own way of handling this, it depends on the conditions they dive in, instructors experience, instructors assessment of the risk involved and so becomes a personal decision.
Because it can be done within standards does not mean it can or should be done.
Please ask the dive shop you will be diving with.
My own take on this.
I will start by saying I teach on a Caribbean island, easy diving, easy conditions and I am an extremely experienced instructor.
Prior to a DSD some training is required, I would take your junior for a pool session and I would ask your referral junior to come in the pool and "help" this gives me a chance to assess both students.
Providing all is good then I am happy to conduct dive 1 of open water and the dsd together.
There are no skills for the ow student in dive one where I can't also supervise the dsd.
Absolutely no one else in the group though ,(sorry, you can't dive with them) as I need to give all my attention to the juniors.
My take on this is different from some others, others may criticize me but as I said, the risk assessment is my personal one.
I should also say that during dive one the dsd stays on the boat while I complete weight checks. Also during dive one I conduct only the minimum skills to meet standards, my focus on dive one is always buoyancy, trim and finning. (Get that right and the rest is easy).
Every instructor has a duty if care to their students. That duty of care is much higher when you have juniors involved. I fully understand the dive centers and instructors that would say "no" to this and I also understand the instructor that does not wish to take responsibility for children diving at all.