I don't know if we were offered the same dates (Cancer Biology here) but yeah, I'm having the same problem
I just got off the phone with Michigan saying they wanted me to visit Jan 28 or a backup weekend on Feb 4, when immediately Emory calls and asks if I can come down on Jan 28 and I said yes thinking I could go to the Feb 4 weekend at Michigan. Then I check the Michigan website and the Feb 4 weekend is crossed out!
I'm hoping it's just because they want people to go to the primary interview weekend. Hopefully, they'll let me come on Feb 4 if I explain it's my only option. Otherwise I guess I'll have to look into an independent visit or Skype interview.