My mother had terminal cancer and my father had dementia. She wanted to move to a nursing home near me and he would not co operate. So we moved her nearer and he went to a residential home near his home . She wasn't t going to get better and his presence wasn't always a help to her. He was accepted for himself where he was. So my decisions were based on what was better for each of them as individuals. The carers thought they should have been together. We thought dad would be upset and confused when my mother died. Mum died after two years illness, in December and we think it was ok to have them separated. Sorry if this is a ramble, just saying our family had them separated and thought it was for the best, we thought others comments were overly sentimental and from people who didn't know the whole circumstances of the family. Nothing is going to be great. Best wishes with your plans.
I guess circumstances and wishes will vary so much according to the particular couple, but I am reminded of an elderly couple we knew, old family friends of OH. They were at least late 80s, no children, she had had dementia for many years. He battled on with it alone, as so many elderly people do - would not have help or any carers in. But things came to a head when HE fell and broke a hip - by this stage her dementia was advanced.
We were startled to find that they had gone into separate care homes, at his request. He confessed to OH that he could not take it any more - she would have given him absolutely no peace - and he wished he'd done it a year or so previously, so he could have had little bit of a life back. He was enjoying life in his residential home, but sadly did not live to enjoy it for more than a few months.
I agree that for the OP, a dual home could well be the way to go. My aunt was in a lovely one of these - residential, specialist dementia and I think nursing, all under the same roof.