When an administrative boundry and a feature like a road or a shoreline run contiguously for a portion of their length, should the one way be drawn over the top of the other or is there a method for them to share points (nodes)?
Welcome to the next holy war 
I don’t know about shorelines, but it seems clear to me that, since the boundary is not exactly the centerline of the paved roadway, it should not have the same points.
I should also point out that one doesn’t exclude the other. When ways are drawn on top of each other, they can share nodes, or each have their own set. In either case, they’re still separate ways.
You probably meant this: with multipolygon relations, you can have shared ways, where the shared way is a member in two (or more) of these multipolygon relations.
But, as NE2 said, the issue is one that crops up with a certain amount of regularity, and never goes anywhere definite. All three* methods are equally valid from a purely technical standpoint, although unnecessary duplicate nodes are frowned upon, while overlapping non-shared ways aren’t yet. And people manage to destroy multipolygon relations fairly often, through both editor issues and misunderstanding, that the overlapping ways is usually the safest bet for now.
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- Overlapping ways, duplicate nodes 2) Overlapping ways, shared nodes 3) Single way, with multipolygons.
- more-or-less overlapping ways, nodes in different locations
Which works , but can be horrible to edit later. I’m trying to sort out a few areas like this and it’s not easy.
Don’t tag for the editor In Potlatch you can thin the boundaries so they’re less likely to be selected.
Method 1, well, makes it hard in JOSM (though not Potlatch, at least not before this flashing node “feature”) to select the correct node. With method 2 you can’t easily keep the nodes in sync when, for instance, joining a new way to the road. Method 3, if the boundary and highway tags are both applied to the same way, makes it really annoying to split in the future if drawing separate ways with better precision. If it’s just the highway tags with a relation, the only problem is maintaining the relation.
Here’s an example of method 4: http://www.openstreetmap.org/?lat=28.64187&lon=-81.37046&zoom=15&layers=B000FTF
Several roads, including Oranole Road (get it?), follow the Orange-Seminole County line. But the only nodes on the county line are at the corners and a few along the way. Changes in road alignment will not affect the boundary, which is defined by the Public Land Survey System grid. City boundaries (none drawn here because I don’t have accurate data) generally follow property lines, and are on one side of the road right-of-way, not down the middle.