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How are these any different than these ones at $13.50 apiece:
http://www.ebrewsupply.com/index.php/internal-hardware/40a-contactor.html
Ah - I see now. Neither circuit will pull more than 40 amp so there are 2 contactors per circuit. I've seen designs where there is 1 contactor for the keyed circuit which means it must be capable of providing the full capacity of the control panel.
If you are wiring it like your drawing you'll be just fine. i.e. your concept will work as you have planned.