Answer by Tester101 for Can a plumbing vent go up and then back down to the connection with the vent stack?

905.2 Grade.

All vent and branch vent pipes shallbe so graded and connected as to drainback to the drainage pipe by gravity.

(2006 International Plumbing Code).

This might prevent you from doing what you want, since your lower horizontal connection would have to slope toward the stack. Not sure if this is allowed or not, but it might be something to look into.