You might be justified in thinking that when you sell one of the houses, you can keep the profit. This is not always the case if you have more than one house with the same lender.
Once being notified of a sale the bank will look at the overall lending. The default position? Funds from the sale to be used towards all the debt(can include debt on another property).
Well what if I need some of the profit?
Most banks will ask you to complete in affect a new application form, provide new proof of income to ensure you can afford the remaining debt. Even if the debt was not used to buy the property being sold.
What if its with another bank?
If the rental property being sold is with a completely different bank, funds can only be applied towards debt at that bank. So it can be a good idea to consider using multiple banks, especially when growing a property portfolio.
