I just did something like this a couple of weeks ago. What I did is as soon as I was done mashing and sparging the first beer, I had the strike water ready for the next beer. Cleaned out the Mlt and doughed in the next beer. By the time the mash was done for the second beer I drained the kettle into a spare turkey fryer pot with my IC in it to start to cool it. Only added about 1 hour onto my brew day.