The operational guidelines for Mortgage Interest Supplement state that :
The purpose of Mortgage Interest Supplement is to provide short term income support to eligible people who are unable to meet their mortgage interest repayments in respect of a house which is their sole place of residence. (my emphasis)
You should discuss the specifics of your situation with your local DSP office who deal with your mortgage interest supplement claim.
The supplement has been closed to new applicants from Jan 2014, and will be phased out for existing recipients over the next 3.5 years.