You what?
What's with all the messing? What are the intentions? Not benefit each other before marriage but benefit each other after marriage?
In simple terms, if a property is held by two people as joint tenants, on the death of one, the property passes automatically to the other. There may be tax implications if they are not married or civil partners.
If a property is held by two people as tenants in common, on the death of one, the property passes into their estate, be that by way of will or intestacy.
If you read up on it, you will see the problems created by people not thinking through what it is they want to do and wreaking havoc down the line.
mf