Its certainly not a PADI standard breach.
Provided the student was used to diving with the twinset and could do all the shut down drills etc i'd have no problem at all teaching someone in them.
What i wouldn't do however is allow someone with a new shiny twinset to do the course as then you'd be teaching the use of that as opposed to the actual course content.
In fact, especially for the deep dive portion i'd like it as realistic as possible - in other words i'd WANT the students to have a redundant air source such as a pony or twinset.
Provided the student was used to diving with the twinset and could do all the shut down drills etc i'd have no problem at all teaching someone in them.
What i wouldn't do however is allow someone with a new shiny twinset to do the course as then you'd be teaching the use of that as opposed to the actual course content.
In fact, especially for the deep dive portion i'd like it as realistic as possible - in other words i'd WANT the students to have a redundant air source such as a pony or twinset.