You will have to set them individually.This is because one set opens and the other set closes.
You will need to block one set open in order to adjust the other.
The easiest is to use a feeler gauge (I believe .020, but my memory fails me and I have no access to any manuals right now. Might try a search of the Mustang and Camaro forums). You can use a dwell meter as well, but the feeler gauge method is easier and is going to be very close to the dwell reading if done right.
I would insure your dist has a vac adv (they come both ways). This will help your raise your cruising fuel economy by quite a bit.
Personally, unless your running a retro rod, I don't see much use for these today. There are quite a few inexpensive options in the electronic ignition arena that will perform better than the early dual set ups, and will require no routine maintenance.
I ran a non vac adv unit on a 65 GT fastback we owned back in the day (wish we still had that car) and wound up ditching it for a stock point style unit, which performed much better.