Can I share a .ipa file (iOS app) among multiple users?
That's a really interesting question.

But first I give you another situation:
If user 2 wants to use an app already bought and downloaded by user 1, the only thing user 2 needs to do is add the .ipa file of that app in his iTunes and authorize his computer with the apple id of user 1. (in iTunes click on 'Store' > 'Authorize this computer')

If two people want to use the same app they don't need to both download the app. They don't even have to both purchase the app. They can share the .ipa file if they authorize each other's computers.

In the situation you present - where user 2 also bought the app but didn't download it - I think he would still need to authorize his computer with the apple id of user 1 in iTunes. That's because I think the identity (apple id) of the buyer/downloader of the app is stored in the .ipa file, but I'm not 100% certain about that.