There are some decent pics on Google if you search "offenhauser slant six dual carb manifold." See below, but here's another one that's a bit larger.
It looks like the stock '64 throttle rod attaches to the rear carb in the same way that it would attach to a single carb. The Offenhauser linkage attaches to both carb throttle linkages, pivots at two points on the manifold, and translates the throttle movement from the rear carb to the front carb.
However, the attachment point with the car's rotating throttle rod is a few inches further rearward than it is with a stock single carb. I doubt there's enough slack to slide that rod backward through the firewall, so you may have to cut and weld it a bit shorter. Or if you have no access to a welder, maybe get creative with a thread die and a coupling nut. That stock throttle rod also includes an arm for the stock kickdown mechanism.