Easiest method for presenting choices

[rusticranch]

We have a database of clinical sites and patients. Some clinical sites have two locations (for instance the UCLA site has two different locations). When a patient is accepted into our clinical study at a site with two locations they are assigned to one of the two locations. What is the easiest way to present the data entry person with the two choices ? Can a message popup based on site number entered telling the entry person to choose the appropriate location? The site file is separate from the patient file but both are linked through relationships. In the patient file we enter the patient stats in various fields and then enter the site number which causes all sorts of fields to autofill (site address, phone number, etc) because of relationships. We do not want to assign two site numbers to sites with two locations.