I think that it would be best to have separate bank accounts for them. Is the intention that both parents go into care at exactly the same time, as self-funders, and their house is then sold, prior to either of them needing a financial assessment from the local authority?
Under those circumstances, personally I would split the proceeds from the house sale between the couple as they have lived together in the same house for 50 years so why shouldn't the wife benefit from the proceeds? They both need their care to be paid for and as self-funders there would a choice available with regards to which home they lived in. With local authority funding they will find a home which meets the wife's needs and is within their budget, which could result in the couple being split up if the intention is to keep them together in the same home if family were not in the position to keep paying a 'top up'. Making them both self-funding for the same length of time would be preferable for them, and half of the proceeds from the house sale going to the wife to pay for her care is unlikely to be considered as 'deprivation of assets' on the part of the husband, particularly as it would have saved the council from having to pay for her care themselves. From personal experience, the longer you can keep away from the need to get involved with social services and the local authority the better as it involves huge amounts of time and stress on the part of the person who is trying to arrange assessments and funding as council departments are not joined up.
If they are both self-funding when they go into care then there is no need for family to pay 'top up' fees. This would only apply if they were unable to fund the full cost of their care themselves and the local authority were funding the care but there was a shortfall between what the council were paying and the care home fees. From what you have said MIL has enough savings to self fund for at least 14 months - is it the same for the husband? If so that will give you time to sell the house prior to any need for a financial assessment.