Why exactly would you want to do that, instead of just making two separate virtual machines?

Anyway, it is pretty much the same procedure as dual-booting DOS and Windows 95, as discussed in Dual Booting DOS and Windows 95 with GRUB4DOS . You will need two separate partitions, and a way to toggle the active partition, such as a boot manager.

If you need to share files between the two VM, you can put each OS on its own virtual HDD and create a third virtual HDD to mount as "D:" in both VMs.