putting certain files on second drive, while mac os remains on first drive...

I have basically all my files on my DRIVE A and nothing on DRIVE B. I want to know how I can put documents and a few other folders, maybe applications, on DRIVE B, while the other one remains the main drive. Everything seems to want to install only on DRIVE A and its making me fill it up really fast and rarely make use of the second one. Anyadvice on what I should do?
-thx

how large are drive a and b?
are they partitions on the same physical drive or separate physical drives?
if they are separate, is drive b a secondary internal drive, or external drive?
and if it is external, is is pretty much continuously connected, or only periodically connected?

My recommendation: Leave your OS and data files on A and back up the data files on B. If A dies, you have your data backed up on B (you can always reinstall the OS on A). If B dies, you have your original data on A. I gives you wonderful peace of mind to know that a HDD failure won't hurt you.

Why would you want to do that?

Use drive "B" has a back up drive like Juan Largo suggested.

Oh and can I combine drive a and b so they work like one disk?

They're both internal. drive a is 120gb and drive b is about 80gb (and maybe a bit slower). Drive b can fit my documents (about 70gb), so I want to move my documents folder to drive b and keep like applications and stuff on a whe I drag documents to drive b, it just makes another copy. I want the actual documents folder located on b