Best practice for making headers for deep (2x8) walls
I am not familiar with 2x8 framing construction (is that even a thing?) but you might look into the way 2x6 construction @ 24" o.c. (a.k.a. "advanced framing") is done. Advanced framing is laid out a little differently than standard 2x4 framing, including different headers, single top plates, no cripple studs, among other differences.

I guess it partly depends on why you are doing 2x8... is this for load-bearing purposes? In which case you really should have an engineer design this highly unusual wall. Or is it just so you can fit more insulation in the cavity, in which case I would think you could just follow a 2x6 framing plan but use 2x8s instead. In any case, double 2x12s seems like a very serious header.