By the sound of this what you'll want to do is to, each time you start up the system, go into the BIOS/UEFI and change the order in which the drives boot so you can pick the one you want to be the first when you want it. There are also methods that by which can get the boot loader to pause a few seconds and let you use the arrow keys to choose between the options for which drive to boot from, but these are trickier to do.
You'll have to install programs on whichever OS they work with, for programs to be used by both OS's you'll have to install them on both drives. While it might in theory be possible to get the OS on one of the drives to look on the other drive when you want to run a program on that other drive it will often be the case that the program needs to be given different settings and such for each OS which it is to be run on.
Personal files could be stored on either drive, and read and written from whichever OS gets booted, although this might need a bit mroe work in cases where one or the other drive has full disk encryption.
Edited by rp88, 15 September 2017 - 12:16 PM.
Back on this site, for a while anyway, been so busy the last year.
My systems:2 laptops, intel i3 processors, windows 8.1 installed on the hard-drive and linux mint 17.3 MATE installed to USB