How can I keep different app versions on two devices synced with iTunes

I have an 1st gen iPod Touch and an iPad 2. The iPod Touch is obsolete enough that most new apps won't run on iOS 3.1. Also, there is a growing number of apps that I own and use daily that have updates that the Touch can't run.

I sync my Touch with iTunes to keep my podcast library up to date. My iPad syncs with iTunes via WiFi.

I use several universal iPod/iPad apps. If I update one of these apps to a version that my Touch doesn't support (because the app now requires iOS 4+), the Touch loses that app. When I try to turn off app syncing for my Touch in iTunes, iTunes warns that it will delete apps and their data on the device. That's not what I want.

How can I sync my iPod Touch with iTunes to keep podcasts up to date, but keep the older versions of the apps around for the Touch and use the latest versions of the apps on my iPad?

Thats a tough one, the only solution I can think of is to have two separate users accounts on the machine you run iTunes on. Each account has its own iTunes file system and this includes the cache of applications, even though both user accounts may use the same iTunes account.

It's a bit of a hack, and not very elegant but here goes...

You would make a backup of your existing iTunes directory from within your profile and place it in a readable location for the other user account.

Log into the new user account, start up iTunes so it initialises your new folder structure for iTunes under that account. Shut down iTunes and copy your existing backed up iTunes account data over the new.

Barring file permissions changes, you should now have two users accounts on your machine that you can control the upgrade path for your applications on.

I know this is an old thread but decided to post this in case it helps somebody. I had deleted some apps from my old iPod (1st gen), and when I tried to add them again, they had been upgraded for my new device and were no longer compatible with the old one. I managed to get them back and keep them using the following steps. This is in iTunes 10.7.

1. Removed newer versions of apps to a safe place, out of the Mobile Applications folder.
2. Copied older versions of apps into Mobile Applications.
3. Deleted library entries for those apps. (I found it helpful to look at the Apps category in list view.)
4. Used Add To Library from the File menu to add the old versions of the apps back to the library.
5. Attached iPod and synced. Apps were loaded without complaint. Disconnected iPod.
6. Removed old apps and their library entries. Put updated apps back in folder and added back to library.
7. Realized that at next sync, apps would be wiped. Dammit.
8. Followed advice in this thread and set preferences to prevent automatic syncing.
9. Connected iPod.
10. Turned off app syncing. I can't remember whether it appeared when I unchecked Sync Apps or when I tried to sync after making the change, but a dialog appeared giving me the option to keep the added apps on the iPod or remove them. Keep!
11. Synced iPod and disconnected.
12. Set preferences to allow automatic syncing again.

Both devices are syncing without any problem, and iPod has retained the apps.