Dual boot between os3.1 and os3.9

[Angus]

I think you could put 3.9 on your first partition and 3.1 on another, and control which boots with the preboot screen selecting the device you boot from.
But 3.9 will probably have some patches that cause a restart (3.1 might too) and this will cause it to go back to the first device, which can be very confusing.  

[Angus]

Oh yes, and how you have the tow OS's to recognise large hard drives could be an issue too, so be careful.