You can apply to M&T at the end of your freshman year but it is ridiculously competitive, think 3.8+ GPA, which for freshman year engineering is insanely high. You can also apply for an uncoordinated degree with wharton in which case a 3.5+ and a very well articulated interest in business in your appliction, maybe also business related clubs, extracurriculars at Penn, would give u a good chance. (Wharton clubs are open to any Penn student so you can join).
All that said , I would say do not stress about it too much. Even if you are not accepted to the uncoordinated dual degree, you could pursue a second major in economics in CAS or simply pursue your engineering degree and take as many wharton/econ classes as you can fit in your schedule. You will have essentially the same recruiting opporunities etc. There are so many engineering kids who do very well in business recruiting and get jobs at top firms with amazing salaries. According to a forbes ranking, Penn SEAS salaries are higher than Wharton’s.
So try for it but don’t get too caught up in this whole thing.
Lastly, yeah with the uncoordinated degree the load is considerably higher and you might need to take an extra semester/year. Generally, as long as you keep you GPA above a 3.0, you will be able to get permission to increase your course load beyond 5.5 c.u.