You could do what most queuing systems do and use the command line render.exe.
Open several cmd windows and type a commandline to start maya batch rendering.
For example the following will render frames 1 through 10:
“c:\Program Files\Autodesk\Maya2014\bin\Render.exe” -r mr -s 1 -e 10 C:\Users\You\Documents\maya\projects\exr_output\scenes\batch_test.ma
This will show you all the general options available
“c:\Program Files\Autodesk\Maya2014\bin\Render.exe” -help
This will show you all the renderers available
“c:\Program Files\Autodesk\Maya2014\bin\Render.exe” -listRenderers
This will show you all the options available for mentalray
“c:\Program Files\Autodesk\Maya2014\bin\Render.exe” -help -r mr
You could write a windows batch script to run multiple maya render.exe commands all from one script.
Or use a proper queuing package that will do all this for you.
You should know that depending on the scene memory requirement running more than one batch render on one computer could be slower than just running one.
In any case running in command line is more efficient than from an interactive Maya session since only one copy of the scene in memory.