If you install the game in different directories you will not be able to play the same game from both installs. If you want to use both mods for the same games the easiest way would be to make a folder under images called halloween, and one called thanksgiving. then whichever one you want to play simply copy (not cut) the images into your game images directory and start the game. If you want to play the other one next time simply copy them over the ones currently in there. Then it will load whichever ones are in the images directory