Training is one of my services and I use my equipment including the mobile device.
If you let them use their own mobile device, they will need to log into your DJI account to be able to fly any distance. Why would you want to do that? But hey, it's entirely up to you. I just shared what I do and now you know why. Do as you wish.
I would only train one at a time and you need to provide your FULL attention and that would be very hard to do with two rookies. Furthermore, according to the FAA you need to be within arms reach to the pilot in command. And, just in case you were not aware of it, you need to have your Part 107 license to do this.