Virtual Sub clients allow you to take parts of the client you set the subs on; this allows you to control certain parts of the machine to backup at different intervals.
Meaning if it had a (C:), (D:) and (E:) drive, or different mounts on Linux - respectively - then you can set each drive to backup on a different day using the virtual sub clients - works the same with with folder directories.
Other edit: I read your post wrong.
All you will actually need to do is set up specific clients on your web interface.
This allows you to set different clients to backup on different days, and you can even use the sub clients if you desire.
You can even group the clients together and have PC1 and PC2 backup one day, and PC3 on another.
All you need to do is inside your web interface, go to settings, select the client, and check the box that activates the separate settings for the client. This allows you to tune the single clients rather than all the clients that do not have that box checked.