For example, in the current case the wife could transfer a 10% beneficial interest (giving rise to a 10% share of gross rents) under a declaration of trust and also confirm a split of expenses 95% herself and 5% husband.
Wife owns 100% initially. She transfers 10% beneficial interest to husband and husband only to bear 5% expenses (not 95%).
Where H and W each possess a beneficial interest in a property and, say, the roof needs repairing at a cost of 100 then the incurring of that 100 has been wholly and exclusively incurred and hence is deductible. The issue is then how much does each of H and W deduct; each deducts the proportion they each incur as per any agreement between the two of them.
I think the issue is not whether the expense has been wholly and exclusively incurred (which it clearly has) but whether on other grounds HMRC could deny all or some part of the expense for H and/or W. As I indicated I suspect if the rental income for H is, say, 1% but his share of the expenses is 99%, say, (taking extremes) then it would seem strongly arguable that H was not carrying on a property business (which I suggest implicitly assumes that it is being carried on with some view to profit).