If you want to do this you need to be able to collect all the runnings into one vessel to ensure that everything mixes well and each batch is equal when split. Assuming you split the total volume exactly you can then split all the ingredients equally as if brewing two batches.
If you are going from extract partial boils to AG full boils and are not already doing so, go get a turkey fryer set up so you can brew outdoors. You'll get a big enough pot and burner as well. If you were planning on doing full boils on a range-good luck, most people cannot get that volume to boil indoors on a stove/range.