We don't have any documentation on getting two different providers working together. You could start by setting up the additional server behavior then modifying the triggers to work with the form element the user selects to choose one over the other.
This selection should occur on the checkout page, you can just add a radio group or a select list for the user to choose from.
Next on your confirm page you would apply the new USPS shipping server behavior and configure it accordingly. Once you have both of them on the confirm page you can them update the triggers to look at the posted value for the shipping selection that comes from the checkout page. If it is one value you would use UPS, and if it is the other value you would use USPS.
Please post back with questions about any part of this that you are not familiar with and I will gladly expand and give you some more specifics.