done a few double brew days.
I use the first waters from the immersion chiller straight into the HLT, usualy get 40 litres or so at a whopping 65 degrees so do not have to heat much to get to strike temp. The rest stored to clean the boiler and for sterailising FVs
If you get the mash tun cleaned out whilst the boil is on from previous brew and grains weighed and in readiness, makes it a bit easier.
I did a double brew day in 8 hours, including cleaning up and pitching. Now that I have moved to 10 gallon brew length I do not need to do so many unless I want to do different beers