Basically having two vessels, for me a 50l stockpot HLT/Kettle and a 38.5 thermopot MT (with a FB) as I already have them. The only element to be in the hlt, and controlled by a PID. The wort going through the hlt from the MT and at the same rate, water pumped into the MT from the HLT. You would soon have wort in both vessels. Step mashing possible by heating the liquid in the HLT, being pumped to the MT maintaining and raising the temp for step mashes. At the end of the mash, closing the valve from the HLT and waiting for all the wort to empty from the MT before closing that off and cleaning it out and moving on with the boil.
Sorry if that's really convoluted. I've done a little diagram in paint to explain;

Is there any reason why this wouldn't work/anything I've overlooked?
My thinking behind it is I already have everything to make the PID, and other than a pump, I have everything else. I've also got a SS catering table big enough for 2 vessels which isn't getting much use. I could perhaps build a permanent setup in the shed with this.
Thanks
Mark