Many times there are communities in your area that offer multiple levels of care within the same location. Depending on the care needs of Mom she may require either Independent or Assisted Living and Dad can be in Memory Care (if needed). If Dad is not a wander risk nor combative it may be possible for them to live in the same apartment in Assisted Living and have help as needed from the Nursing staff. An assessment from the senior living community would determine the best course of action.
I would recommend a community that offers both Assisted Living and Memory Care within the same building. Speak to the community directly about your Dad and his needs, sometimes, a person with Dementia is able to live in the Assisted Living portion of the community as long as they don't wander or exhibit behaviors.