If you're diligent enough at checking your PC account, then as soon as the first night's miles/points post, immediately change your earnings preference to the other. Hopefully, by the time the 2nd night posts, it will see your new preference and post accordingly. I've done this before but I don't think it was on a back to back night. There were at least 2-3 nights in between so I had some leeway with changing the preference. Still might work though.