The same as async_ffi_single, but for two output values.
Should be used for any async executor accepting a callback of type:
fn(*mut c_void, ResultFFI, ConstStr, ConstStr)
This should be used for any async executor function accepting a callback of type:
fn(*mut c_void, ResultFFI, ConstStr)
you can access the cb and extra pointers via the CallbackData accepted
Note that they should not escape the 'a lifetime of the CallbackData<'a>.