Because one day when I call the bank, they were asking me which property I want to put as main residence, which makes me think they want to put the other one as buy-to-let rate.Why are you asking this question now? assumption is that these are existing mortgages and no bank has raised any issue over them. If you continue to meet the payments the bank will have no issues with whether you live in both properties or not!!
You are not renting out the 2nd property so I assume that you are not in breach of any loan conditions. They cannot arbitrarily change the rate on a facility unless you deliberately misled them when you took out the loan!
Or most likely they wanted to know where to send any correspondance such as your annual mortgage statements.Because one day when I call the bank, they were asking me which property I want to put as main residence, which makes me think they want to put the other one as buy-to-let rate.