Lots of grain, two mashes? Partigyle?

I only have one 5 gallon cooler MLT and have generally only done 12 lbs. of grain in it at the most before. Would there be any way possible for me to get enough grain or a process together with just this MLT that would let me do a strong beer and a sesion beer in one day to get two 5 gallon batches? Two seperate mashes and lots of seperate pots to keep the other wort warm?
Start mashing in another vessel and pour it into the cooler to wash/sparge? Any great ideas here?

In order for you to do a Parti-gyle you are going to have to draw of 5gallons of wort from your first runnings. I'm guessing you could add water, but that would throw off your OG, so your calculations maybe really complicated. I think a parti-gyle would be tough with the size of you mash tun.

I know that back in the olden days, they used to mash in 1 vessel and sparge in another (there are risks with oxinating the mash), so there is that possibility if you wanted to research that I think that would be your best bet.