I think the only reason it really works is because when Obi-Wan says "a pupil of mine" about Darth Vader it gives the impression that he trained multiple people.

I'm not sure it would be possible to add another apprentice. The only thing I see it working for is to show that Anakin is truly a full fledged jedi. Having him and Obi-Wan working together at the beginning of RotS was a missed opportunity to see Anakin acting on his own. When you keep seeing them together, you start to doubt Anakin's ability to ever strike out on his own.