Just say you were wanting to rent a flat but the landlord asked that you have a guarantor, The guarantor needs to be a homeowner but is a joint home owner so do you need both homeowners to be guarantor or can you have one without the other one knowing.
The guarantor is simply saying "I'll pay up if the tenant doesn't". That offer has nothing to do with his/her property ownership (joint or otherwise), so only his/her signature as a guarantor is required.
make sure the guarantor knows that in the case of default, the landlord really will come after him ( he has a home see, so there is an address and equity and so on )