I know some people who did two internships, the ones I can think of were sponsored students from big name MC companies. They also seemed to focus on start-ups or smaller companies. My advice is dont try to do two internships...10-12 weeks is a lot shorter of a time than you realize. It takes a while to get up to speed with a company and your project. If you are switching careers you will get a hell of a lot more out of focusing on one project and one company. If you are returning to an industry or a sponsoring company you could do two internships just for the fun of it.
_________________
Kellogg Class of 2010...still active and willing to help. However, I do not do profile reviews, don't offer predictions on chances and am far to busy to review essays, so save the energy of writing me a PM seeking help for these. If I don't respond to a PM that is not one of the previously mentioned trash can destined messages, please don't take it personally I get so many messages I have a hard to responding to most. The more interesting, compelling, or humorous you message the more likely I am to respond.
Find out what's new at GMAT Club - latest features and updates