Is it better to have a strong letter of recommendation from a professor in a different area of study or a weaker letter from the same area of study? I want to apply for a graduate electrical engineering program next fall. I have one engineering professor who has written letters of recommendation for me in the past, however I only know her from more generic courses which I didn't always do well in (hence why I know her from office hours).
The other letter I could get would be from a business professor. I took two courses with him which I got A's in, spoke to him about a startup I cofounded, and was in a student community he chaperoned. He could likely give me a stronger letter of recommendation, however he was a professor for business management (specifically entrepreneurship), and knows little about engineering.
Although I do have one letter from another engineering professor I'll submit regardless (and one from another cofounder of the startup), is it better to have a stronger letter of recommendation from a professor in a different department, or is it better to have a weaker letter from a professor within my area of study?
If you have your engineering side covered already with strong letters, you can get a strong letter from outside of the field.
However, you are applying to an engineering program, so first and foremost, you must make sure that the program managers understand that you are strong in engineering, not business. It is no harm, of course, to demonstrate that you also can do business.
I cannot tell you whether just one support letter from your core field is sufficient, probably you need at least two.