I just didn't pay any attention to how many stitches I should have cast on in the first place. I have no idea why I was so inattentive or why I got the number wrong on both sides. Second time around is going fine and I have almost one side done.
I did consider doing the two sides at the same time, but figured there'd be more brain work required. I do a lot of double-knit style knitting, but shaping doesn't come automatically for me in that mode.