The two fields are unrelated. Marketing is a general term. Ask yourself if you want to do sales or advertising, the two branches of marketing.
Starting salaries vary widely. If you are creative and get hired by an ad agency, you could start at $40,000.
Instead of worrying about starting salary, pursue a career in the field that interest you the most. Once you learn the ropes, then figure out how to make money at it.

I wouldn’t do it. Nursing knowledge won’t help a marketing career and vise versa. Decide what you really want to do and then pick a second major from there if you are so inclined to do so. I would suggest Spanish as a second major or minor to anything. I don’t know the area you live in, but knowing Spanish will earn points with any possible employer because of the increasing number of Spanish speaking people.

It’s a brilliant idea. Who would be a better marketing manager at a biotech, medical products, or pharmaceutical company? Someone who majored in Spanish or someone who understood nursing. The health care industry is growing rapidly and they’ll require marketing people. But, I would do some more research about marketing positions in the health care industry. Oftentimes, the best marketing people are involved in the nuts and bolts of an industry for 10 or more years before they start working in a marketing department.

Marketing/Advertising doesn’t pay very well out of college.
However, with your double major maybe you could work for a hospital/clinic/pharm sales and make more – Just a thought.

I would think that your study works fine. Companies such as Nestles, Johnson & Johnson certainly needs people like you. You could market their products to maternity hospitals or nursing schools and homes for the aged to name just a few.
That let you cover both marketing as well as brand management/advertising.