wirecat, for other readers' reference, there are some example online wiring diagrams of 'non-simultaneous, two-wire' {page 4} and 'simultaneous, 4-wire' {page 6} electric water heaters.
http://www.hotwaterproducts.com/BWC/Parts/Pdfs/Electric%20Wiring.pdf I don't know if your heater has the exact same wiring, but there can’t be too many variations of two heater elements with two thermostats.
