Manu blend between virtual camera A/B

I was really hoping to find a way to do this but after digging quite a bit and searching around, I'm not seeing a great solution. I've wanted to simply set up two virtual cameras and manually be able to blend from camera A to camera B via a blend value T. If this is possible and someone can explain the means of achieving it. I would greatly appreciate it. If not, it seems like it would be a very useful addition.