You should't try to the same virtual name for both (think about it, how could that work?). Lets say your 18.2 version was www.something.com/rachael - set up the 19.1 version as www.something.com/rachael19 (or whatever) and then the customer can run either one depending on what they are doing. Eventually you'll want to retire the 18.2 version and then you can set the 19.1 version back to just www.something.com/rachael.

To "fix" what's messed up - I'd stop the apps (both versions), remove the webshares in IIS and then use the 18.2 and 19.2 Studios to (re)register the webapps (see the link in the Start Center). This will not only set the webshares correctly, but regenerate the global.asa files and fix the index.htm files so they are correct.Then once you have them both running side-by-side in your development environment you can redeploy them.