My understanding is you can have only ONE primary

house any time. You can have "second home" which you don't rent out, but "second home" is not "primary house". If you really live in one house for 3 days and the other for 4, then the house you spend more time (even if difference is trivial) has to be deemed as "primary".

However, for the loan matter, what if someone change his mind after purchasing the future primary house? Is there a document asking you to report once you don't live there as primary? I don't know.