Can't share wirelessly BOTH internet and files.

Until past week, I shared both internet and files from my iMac (Mountain Lion, 27 inch, late 2009) to my MBP (Snow Leopard, 15 inch, late 2010). Suddenly something went wrong (a BlackBerry of a friend couldn't connect to internet, so I turned off the protection to see if the problem was that -it was-, and when tried to set up things back, it all messed up) and since then can't do both at same time: if I create a computer to computer network, MBP can access files and print through the shared printer on the iMac, but doesn't get internet. And vice versa: if MBP has internet, it can't access the files or mount the iMac (in fact, doesn't even appear on the network). Obviously, sharing files option is on.

I tried deleting the network prefs and also creating two different locations (one for the network, with WEP security and other for the internet sharing, with WPA2 Personal, since there are no other options rather than "unprotected"). None of this worked.

Any help will be greatly appreciated.