In more detail . . .
If two people were to purchase a property as 'tenants in common', then each would own a half share in it. One person couldn't obtain a mortgage on the whole property because he/she would only have half of it to offer as security to the lender.
If two people were to purchase a property as 'joint tenants', then it would be their 'partnership' which owned the whole property. If a lender was crazy enough to provide a mortgage to one of those persons, and that person then died, the house would automatically become the property of the other joint tenant, leaving the lender with no security against the loan which was due for repayment from the borrower's estate. (i.e. unless the borrower had enough money in the bank to pay off the loan, the lender would lose out).
It ain't gonna happen!