Maybe she is constipated. Go with her when she goes out to go to the bathroom and make sure she is going. If she isn't you can get medicine from the vet to help her go normally.

that means they have pin worms or round worms.im serious look it up.they tend to release eggs which make an animal rub its rear on the carpet.sorry to break the news

Call a vet.. Or better yet, take them in!

As I'm reading this question...you have two different dogs...both doing the exact same behavior late at night. They are not rubbing their butts on the carpet, they're butts are not actually even touching the carpet...and at any rate dogs who rub their butts on the carpet, or grass are generally not worm infested, but need there Anal Glands to be EXPRESSED (emptied) because they (the glands) have become impacted! But, as I read this, THIS behavior is not the case...they aren't rubbing their butts at all! Interesting....does it happen at the same time and does it happen EVERY NIGHT? When did they start doing this...when did you move into this house? I realize THIS is off the charts...but my first thought....assuming I've understood your question and description is that your Dogs are sensing SOMETHING OR SOMEONE...that YOU and I can't see. I realize it sounds nutty...but it wouldn't be the first time that an animal has reacted to something that humans cannot explain in any logical manner. My partner's Grandma (dead for over 20 years) evidently lives or visits us. The dogs rarely react to her but our CATS do. I've watched Sipper stand and lean up as if she was being petted...Sipper also yowls several times EVERY NIGHT at the exact same time. Our Ex roomie used to have problems with her tv remote being used to change the channels, late in the evening...the channels were always changed to the SAME old TV shows and old Game Shows that my Partner's Nanny, liked to watch when she was alive..... I've seen something and when I described IT to my partner she freaked out and said, "That's my Nanny!" I never met her Nanny, I never saw a picture of the woman, and what she was wearing was her typical nightwear...which I had no clue as to what THAT would have been...no one told me about it! So...without seeing your dogs doing the behavior, I can only consider that you may have a spirit visiting....I don't know anything else that might logically cover this!