Ummm...your post is a little vague. You don't specify which input devices you have in mind or what games you are trying to play. :^\
Pretty much all of the games that I have will operate correctly with more than one input device activated. If you want some sort of universal interface that will give you configuration options for several active input devices, maybe you should consider getting something like USBOverdrive.
http://www.usboverdrive.com
But I think Gamesprockets will give you all the real control you need. You'll just have to keep experimenting until you get a device combination that appeals to you.
I have several input devices in my arsenal: a Saitek P750 gamepad, a Thrustmaster Firestorm gamepad, a Trackerball trackball, and, of course, my keyboard. I use different combinations of devices all the time while playing games, and everything works fine, even without USBOverdrive.
Hope this helps.
Tristan