Really depends on your cooking 'style' and expectations, and FWIW I don't see that this is particular to the CDT in any way.
When my wife and I backpack together, I just carry the same 850 ml pot/mug that I cook in when I'm solo (and using a stove at all). It's big enough to heat two cups of water at a time, and we don't eat enough generally to need more. The couple of times we have, I just heated water twice.
On a thru-hike, however, indeed your calorie intake goes up and a larger pot might be more efficient, but ... seems to me that if you're thru-hiking (?) the CDT you would already have enough experience to just figure out or wing this!
I personally would only start a long distance trip self-contained in every way, not sharing stove or tent or whatever.
Maybe in this case a big enough light titanium pot to heat the water you both want would be more efficient, if the odds look good that you'll stick together for a good while. But stuff happens. My starting CDT hiking partner hurt himself and had to leave the trail before we got out of Montana, and I hiked Wyoming and Colorado solo until I partnered up with another guy in northern NM.