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?
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.