I have received summer internship offers for two locations within the same company. One offer is roughly 2.3x the salary of the other, but I would prefer to work with the people at the lower paying location. How can I negotiate this effectively? In particular I would really appreciate advice on how to word the email to the internship coordinator (or perhaps a phone call is better?). Should I contact the HR people at both locations, or only the location that I want to work at? 

Another issue that I received an offer to work with a specific scientist. A couple days later, another scientist at the same location emailed me to set up a phone conversation. These two scientists often work together and publish together quite a bit. How do I inform the second one during the interview that I have already received an offer from his colleague, but would be open to being co-mentored or something like that?