Split batch

[Grainydays]

I’ve been brewing extract with grains for years. I recently received an 8 gallon gas one brew pot so I’ve been subbing out extract with more grain. I want to make a really hefty stout. Can I split the recipe, cool it quickly, hold the temp at 70 and brew a second wort get it to 70 degrees then gently combine and pitch? Would boiling time still be 60 minutes and would I use 3.5 gallons to start each brew. I’ve seen similar posts but none that answer all these questions.