How does this differ from a sole tenancy?
It is identical to a sole tenancy but it is important to bear in mind this definition of the "tenant". "The Tenant" is a legal entity of its own, being any or all of the tenants jointly. Therefore, any obligations not met by "the tenant" - ie all of you - can be attributed to "the tenant" - ie any of you.
Examples would include non payment of rent - ie if the couple agreed between themselves to pay half of the rent each, then if one doesn't pay, "the tenant" is responsible and the one who had already paid their 'share' could be sued. Likewise, if there is any damage caused - that cost is paid by "the tenant" - ie all of the occupiers.
Individual arrangements between tenants as to responsibility do not affect the tenancy agreements validity and would need to be sorted out between to individuals, possibly including court action.