You could also pull one of the reels, put it in a light-tight container (a second developing tank, say), and develop the first roll with the compromise time. You'll then know which roll you haven't yet processed and be able to process it with the correct time.

I'd only do this if I were 100% sure that the temporary container is light-tight, though. Even a developing tank might not be light tight if it's not completely assembled (say, if you put a plastic reel in without using the central rod).