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Is this what you are asking, just to clarify?
You use one google account for your gmail.
You want to use a different google account for gtalk.
I believe so, but I can't verify. All you should have to do is go to settings>accounts and sync>add account
Then add your separate google account. Now I'm not positive, but from looking at it, you should be able to disable anything with the 2nd account from syncing, so you won't see the 2nd account's mail in your gmail. After that, open the gtalk app and see if there is even an option of which account to use to sign in.
Let me know if that works, I'm sort of interested.
**Forgot what forum I'm in. Just saw you are on a Hero. My directions are based on 2.0/2.0.1 on the Droid. Is Hero running 1.6?
I found something on another forum (Changing GMail/Contacts/Calendar account on HTC Magic/Android 1.5 - Android Forums) with a Magic/1.5 which may or may not help. It mentions you can do it for Gmail and for your Messenger. Hopefully you can clear one without having to clear the other.