Mac OS X中的多个独立R会话

I need to run multiple R sessions, and hope that they can be performed in different R sessions. In Windows, I am able to open an arbitrary number of R sessions, and run different codes in each session (both RGui and RStudio). However, in Mac OSX, neither R.app nor RStudio would allow me to open multiple independent sessions -- I have to wait until the first set of R codes are completed in order to run a second set of R codes.I need to run multiple R sessions, and hope tha