Multiple iOS Devices Problem

[JUiCEJamie]

Hi Guys!

I'm having a little difficulty keeping my Library of iOS devices how I want them.
I own an iPad2, iPhone4 and 3GS. But my partner also has an iPhone4 and an iPod Nano.

My music collection is fine, we both buy CDs etc, and that's not a problem.

But, APPS... How can I organize iTunes so when I plug in my iPad2, it syncs correctly, keeps the apps I want on, without placing all my iPhone4 apps on etc etc..

It seems as though each iOS devices becomes identical.
Does anyone have any tips on how to help?

Thanks!