Hopefully I haven't overloaded myself, but now that I am halfway through my Diploma I've signed up to do an Undergraduate Certificate as well next year. It will mean I have 1 1/2 semesters where I have three subjects (which is 3/4 of a full-time study load, on top of full-time work) but I think it's doable. Fortunately a few of the subjects have reading lists at the library so I can do most of the reading in advance which will help.
If I can manage that I will feel a lot more confident about my plan to sign up for a double degree course the year after.