The twin gig modules are only compatible with the 8-port 10 Gigabit Ethernet expansion modules. See figure 4 on the product data sheet. This is also mentioned (though not as explicitly as I would like) in the Installation Guide.
You cannot use them in one of the eight built-in 10 Gig ports. I only found this out conclusively after opening a TAC case.
I made the mistake of assuming I could do that a couple of years back and it still bugs me. :)
