Sending pre-application e-mail to two professors who are close friends but at different universities
There is no issue here. You're not going to burning any bridges or make anyone upset if you talk to other professors. Your job right now is to find the best university that will take you. The professors at these universities understand that. If they don't, you don't want to work with them.

Personal story time: When I was applying to grad school, I ended up chatting with a husband and wife who were professors at different universities. They had different last names and didn't let on they were married so I didn't find out until another professor told me. I thought it was hilarious and they didn't care that their spouse was also trying to recruit me.