On each floor I have a tub and a sink back-to-back with a 2x6 wall in between. Can I run a single 2" pipe as a wet vent from below the slab to the two fixtures, then up the wall to the second floor fixtures, then up to the attic and main vent? If not, would a 3" wet vent work? Or should I run a parallel dry vent from the first floor up through the second floor, then up to the attic?
According to this article about wet venting at h2ouse.org:
Although wet vent plumbing is used to vent two or more bathroom fixtures at the same time, the fixtures all have to be located on the same floor of a particular building. They cannot be on separate floors.
They also point out:
It’s important to remember that codes will vary by city and state. The guidelines listed below are typically universal to both the United States and Canada.
So you'll want to double-check your local building codes to confirm.
Here is an example of a code that asserts that the fixtures must be on the same floor.
Wet-vented fixtures shall be within the same story; provided, further, that fixtures with a continuous vent discharging into a wet vent shall be within the same story as the wet-vented fixtures.
A bathroom group located on the same floor level shall be permitted to be vented by a horizontal wet vent where all of the conditions of Section 908.2.1 through Section 908.2.5 are met.