Ok, not sure exactly what you're asking but here's my experience mashing in both my brew kettle (using the brew-in-a-bag method) and using an actual mash/lauter tun (a ten gal cooler w/false bottom). When I used the BIAB method, I used my 7.5 gal kettle as my mash tun. It held heat fairly well if I wrapped it with a towel. Since ~7 gal is rather small for a mash tun, a few times I had to split the mashed into two vessels, which I think is what you're talking about here. I used my 5 gal kettle as a second tun, wrapped in a towel it held temps just fine. Splitting mashes also worked just fine for me. I found that when mashing in the kettle, minimizing the headspace (by filling it as much as possible per recipe) really helped stabilize temps.
I now use a 10 gal picnic type cooler with a false bottom for mashing. With the cooler preheated, temps much more stable than a brew kettle, and the process of mashing and sparging is much smoother than doing a BIAB in the kettle. It also holds a lot more grain, so there's never a though of splitting the mash.
I'd say if cost isn't a worry for you, go with an actual mash tun of some sort.