Any care home can decide not to take any potential resident and any decision to accept one will be dependent, not only on finance, but also whether the disabilities the potential resident has match the conditions that the care home can care for.
For example, many care homes take people with dementia, but only specialist units will take dementia patients with severe behavioural problems. When one of my uncle's was in care the mental condition of the man in the next room deteriorated suddenly and he physically attacked my relation. The care home he was in did not have the specialist facilities to care for people with severe behavioural problems and he was swiftly transferred to a care home with the necessary facilities.
Some homes will also have waiting lists, although in a case like yours they may well let your DF queue jump.
Do your parents want to share a room or have two separate rooms? Another aunt and uncle of mine went into (self-funded) care together, both with dementia, and were insistent that they had to share a room. Their entry into care was precipitate and the only care home we could find with two vacancies had them as two single rooms. When, on the first night, DU insisted on sharing his wife's bed and she got distressed when they tried to move him, the care home moved both beds into one room and they used the second room as a day room. It took us several months to find a suitable care home with a double room.
If the care home cannot accept him, you could move both of them to a care home that can accommodate both. In this case you would have to balance any stress changing care homes would cause your mother against the benefits she would gain by having her husband living with her again.