I have a flight coming up tomorrow where the outbound is on Airtran and the return is on UA. Both are within 24 hrs. One ticket. I am checked in on Airtran. United refuses to let me check in with this error: "united.com check-in is not currently offered for flights that are operated by another carrier. Please proceed to the airport for check-in." Tried the website and the iOS app, same error (so presumably backend). Any workaround?