So I did what I normally do when I ask for advice on internet forums and receive intelligent, well-informed, and 100% accurate information/advice: I ignore all of it and just do what I had planned on doing all along
In this case, that was to keep the dev volume at my normal 500mL for 2 rolls, and just increase the amount of agitation (as per James' suggestion). And it seems to have worked! The top roll came out perfectly fine, no small band of under-development. I imagine that dev volume and frequency of agitation might work like aperture and shutter speed in this case, so if I increased one I could keep the other the same. Or something. At any rate, it worked, problem solved, thanks for the great advice, and sorry I ignored it all