If one of the tenants stops paying part of the rent, the landlord could pursue whatever default remedies are applicable under the lease. Among other things, the landlord would have the right to collect any unpaid rent or other monetary damages from either the remaining tenant or the departing tenant or in some combination from the two of them. If the remaining tenant ended up having to pay more than such tenant’s proportionate share, the remaining tenant would potentially have a claim for the excess against the departing tenant.