TRACON Controller providing OJT elected to partially take over the position to demonstrate a seldom used technique; ultimately resulting in a near loss of separation event.

Narrative

Training session on Final; I was the trainer. There was a full down wind; when Aircraft X came on frequency. The Aircraft X asked for Runway XX; which meant I had to take him over the top of Runway YYL and Runway YYR finals. This is a common practices. The trainee was unsure so I told him I would do it and he could watch. There were two aircraft on final at 080 on a Visual Approach. I turned Aircraft X base over top at 090. Prior to that the developmental cleared Aircraft Y for a Visual Approach to Runway YYL off the downwind. The Aircraft Y started base and descended out of 090. Aircraft Y turned a short base and started to cut off the Aircraft X. I don't know if separation was lost. But as Aircraft Y turned toward Aircraft X; he was able to get visual separation. It was close to 3 miles. There was divergence and visual separation but at what point I am unsure. Both aircraft continued to the airport without further incident. My recommendation is during training; if I am going to take over; I should take everything over. I was still letting the developmental work other aircraft. This caused me to lose track the aircraft he had cleared. If I was working everything this would have never been an issue.