Running a GUI application on your Linux machine as a different user
Update 2015-03-23: It seems that the program get privileges to both accounts. If I start Firefox with it, it can read both from my account's files and the the account that sox runs it as.
sux seems to do the job. If you are on Ubuntu:
sudo apt-get install sux
sux other_user_name gui_program_to_run
You may need to include the full path to the program you want to run, e.g. if is a Windows program to be run under wine
sux other_user_name wine /path/to/program
Haven't tried without full path though.