Not sure how you're determining how to join the other shapefile, but if it's based on name you could do something like below:
import arcpy
arcpy.env.workspace = r"C:\data\vector\USA"
shapefileList = []
for shapefile in arcpy.ListFeatureClasses("*"):
shapefileList.append(shapefile)
arcpy.env.workspace = r"C:\data\vector\USA2"
for shapefile in arcpy.ListFeatureClasses("*"):
for shapefile2 in shapefileList:
if shapefile == shapefile2:
arcpy.JoinField_management(shapefile, "JOINFIELD", shapefile2, "JOINFIELD", ["ID, PIN, ADDRESS"])
You can also use the Join Field tool rather than doing a join and then export. This will save you a step.