Don't think of a back-up as a back-up "degree", so much as a back-up plan. Any degree you get is great, but if you're going to be marketable in ANY field you've got to maintain currency in that field. No matter if your degree is math, engineering, computer science, whatever, if you don't keep current in that field, maintaining current field knowledge, up-to-date information AND current contacts, you're not going to get many job offers out of that degree. Your marketability in the field goes down with every year that passes that you don't work to stay current in the field.
Get a degree in whatever you want. If you really want a back-up, think of it is a back-up PLAN that you must always be keeping up to date.
Get a degree in whatever you want. If you really want a back-up, think of it is a back-up PLAN that you must always be keeping up to date.