Will your university even allow you to take both courses at the same time? They're normally taught as a sequence, with the first semester (mechanics) being pre-requisite for the second semester (E&M). In fact, I'd be kind of surprised if they're actually offered at the same time in the summer, unless you're at a very large university. They're most likely offered during consecutive half-semesters, with each course taking six or seven weeks. I've taught that sort of course: an hour and a half of lecture, five days a week, and two labs per week. I called it "Firehose Physics."
E&M does extensively use concepts from the first semester. As AC noted, you'll be using vectors right off the bat in E&M, and when you get to electric potential (voltage) and circuits, you need to be familiar with energy and power.