To properly implement this you will need to have two new shipping rules for the flat rate charges.
To get the new rules to work you can add to the select list that you already have in place. The next step would be to check this select list on the confirm page. If the value is set to one of the flat rate shipping options then you can set a session variable. You can set the session variable to one value for the regular and another for the international.
In the new flat rate shipping rule you will check for the presence of this session variable then the value, if it is the US flat rate value the you apply that shipping charge, and if it is the international you would apply that charge.
Along with this you will want to alter your other UPS shipping rule to not trigger if this session variable exists. If you need some specific pointers on this please post back with some questions about any part that you need help with and we will be glad to help you get it worked out.