I like the workaround provided by kweathe with having a unknown computers collection for both architecture types, however, in my environment, I have consolidated the majority of our task sequences into a single task sequence. The problem this creates is pretty obvious; you can only assign a single boot image to a task sequence. The only option, I suppose, is to duplicate the task sequence and very carefully maintain them so that they remain identical except where they must differ. Oy vey.