Income and expenditure for a married couple visit to the UK

Looking at the Standard Visitor Visa form*, it seems very clear that they are looking for information on each applicant individually. There is no such thing as a joint application, although you can reference each other's applications in the notes. It is entirely possible that one application from a married couple would be approved and the other denied. For example, in box 5.15, "What money is available to you for your trip?", you might answer "Joint account containing >£3000 with my husband, also travelling, referenced in notes". For other boxes, simply split the answer based on what you personally pay/earn. For expenses from joint accounts, split them according to what percentage of money paid into that account is by you.

* Yes, I'm aware that this isn't the form OP will be filling out, it's actually specific to applicants in North Korea, but I'm working with the assumption that the content is identical to the online system most applicants use. Looking at the form, this makes perfect sense, there's no reference to NK on there, and all of the questions seem normal.

To be honest, it doesn't really matter unless the figures you put down would cause suspicion by anyone who bothers to look at them. It's not like they are actually going to call your employer and check. I would combine the incomes and put the same number on both forms, since a higher number is less likely to be examined.

I think the primary reason for asking these questions is to see if anyone actually puts down that they don't have adequate funds for such a trip, and then that would lead to a follow up such as "why is this person traveling here if they cannot afford it?"

In the extremely unlikely event that a) the combined income is the incorrect way to fill it out and b) someone actually notices and confronts you about it, then just apologize and explain that you misunderstood.