Well, if I read this right, they just mainly switched which was the primary HDD to boot up. You can have multiple operating systems on one disk through disk partitioning though which is a separate issue entirely. Basically its like this.

In this situation, you have a computer with 2 Hard drives; both with operating systems. Now one boots up in a more complicated OS (Linux based) while the other is very family friendly (say Windows 7). The first thing you do is turn on your computer. What comes up next is your BIOS which you go into and specify which hard drive you want to be the "primary"drive. From there, the choice is up to you.

Its not really a hard thing to do, it is just setting up different OS's and setting up different hard drives. That is the creativity of computers, you can do many things with them. At least, that is what read from what you said. Does that answer your question?