[DanEC]

When I started my ERA a few years ago I asked a similar question regarding idle tuning on my dual quad 428. It was recommended to close the primary blades on both carbs and open them equal turns of the curb idle screw to a rough-set idle speed. Then set the idle mixture in both carbs, which requires a bunch of back and forth but isn't overly difficult. A vacuum gage or a tach can be used for this depending on which you prefer to work with. After a couple of rounds it will probably be close and further minute adjustments probably won't make much difference. Once the idle mixture was set and balanced between the two carbs for the highest vacuum or rpm, then make equal adjustments to the curb idle screw so that both carbs are equally contributing to the desired idle.

Can't swear this is what Ford intended or the best approach but I've been using it and it seems to work fine on my FE. Now I've been helping a friend get his 69 Hemi Roadrunner going and it has original Carters on it and the front one (secondary carb in this case) doesn't even have idle screws and I therefore guess an idle circuit. But if both of your carbs have primary idle circuits, IMO they both need to be adjusted for mixture and idle speed.