Buy tray-less HDD drive bay and another HDD. Install 2nd OS on new HDD. Swap out HDDs and therefore OS as you please. Enjoy.
Disadvantage: Costs money, it's more work to switch OS and doing it often will wear out the disk connectors, no data sharing.
Advantage: No fiddling with boot loaders, you can't fuck up one OS from within the other, no data sharing (yep, that's on both lists).