Departments don't like to fund students who don't belong to them, so you need unambiguous departmental links (a). If your focus is divided or unique, your financial and social support networks will also be divided, or possibly non-existent. One undergraduate at Swansea University reported himself unable to study the mooted Gender course because of these conflicts,and so did English and Gender - a much narrower study because of the limitations of overlap. As any undergraduate doing joint honours will know,each subject demands two thirds of your time, so dividing your time and putting in the work is more challenging (b). At Oxford University, a German and Classics degree was a five-year Bachelor's - clearly enough work for a Master's, but not of either subject, and so it was not recognised (c). Even if you can divide your time, multitasking decreases your productivity, so dividing your attention between subjects can be tricky: trips may tear you away from the lab or library and waiting for confirmation from different people retards projects (d). But do remember that when you were in school you probably studied twelve or more subjects at a time - and coped then. This is what I mean by narrowing down.