They are not both advance units. One is a retard unit and that line connects to one of the ports on the tree that comes off the manifold. Quite a few people leave this disconnected, blocking off the vacuum line. The advance unit connects to a port off the carburetor. Follow the link and you will get to a tech writeup available on the OGTS website, that explains what you need. Looking at the distributor, the advance unit is to the left of the condenser, while the retard unit is to the right.