For Mac users out there, you may already be aware of this but I thought I'd post a note about it in case some don't.
I have been experimenting with rendering multiple MS movies simultaneously. I've been able to render three high-poly movies at once by changing the system default using the command prompt allowing you to open multiple instances of a program. This has been a huge help for me as I use lots of green screen layers of a movie for external assembling, enabling me to work 3X faster.
I believe on a windows machine, 32 bit mode and 64 bit mode are separate processes and the OS switches from one to the other when 32 bit processes are needed. On a mac it seems that new instances of a 32 bit application creates new virtual 4gig memory allotments for each process, rather than share a single 4gig memory allocation.
I am doing this on a macbook pro with a 1 gig dedicated nvidia 650m card, 16 gigs of RAM, and quad cores running Yosemite.
Here's the line command to enter in Terminal:
open -n /Applications/ApplicationName.app
[I'll add that I know this behavior is not officiall supported by MS, so try it at your own risk, and make backups!]