The primary mortgage for a certain property is the first mortgage, and if you haven't borrowed up to the maximum amount that is loanable, it is possible to get a second mortgage. Of course, the interest rate would be higher, but it could be helpful in case of an emergency
Whether you have one or two houses, if you have only one mortgage on a house, it is always considered the first mortgage. If your financial situation allows it, you can get multiple mortgages with good terms.
When you first purchase a house, if you haven't borrowed up to 80% of its value or if the value has increased since then and now 80% is worth more than it was at the time, a second mortgage is possible.
Getting a second mortgage is easier and has much lower interest rates than obtaining a business loan. Therefore, it can be considered for starting a business or for a major event such as a child's wedding, where a large amount of money is needed.