Hello,
I don’t sure which is the best/correct way to map a two-level tunnel like of the next image image: 
One of the roadways is exactly on top of the other, but it isn’t seems a good idea to connect nodes of two ways with different layer=* value, according the second paragraph of Key:layer wiki article.
Thanks.
I was going to suggest generalising from the San Francisco Bay Bridge, but something seems to be seriously wrong with the map there. The data for the bridge seems to be missing and the rendered tiles seem to be showing side by side carriageways, rather than the stacked ones that exist in real life.
In Maastricht (NL) mapped as:
https://www.openstreetmap.org/query?lat=50.8406&lon=5.7173#map=16/50.8406/5.7224
@whturner: Could you identify one of the objects in Masstricht, as I can’t see any obviously stacked tunnels.
@jptolosa87: sharing nodes is definitely wrong, as it indicates connectivity. Although it is a pain, I think you need to put layer tags on the nodes as well. You can use filters in JOSM, to allow you to access a single layer at a time.
You can also stagger the joins, as I did for this spiral ramp: https://www.openstreetmap.org/way/467030041
As I remember it, simply filtering ways based on layers prevented JOSM snapping to untagged nodes.
I wouldn’t try to map this with iD.
This way on layer -1 and this way on layer -2. They were drawn with a horizontal distance of about one meter.
When inside a tunnel look how it is done in buildings, drawing footway in underground railway stations.
There they use level=
Buildings have building:part and building:levels
Bridges have man_made=bridge and bridge:part
When someone decided to use man_made=tunnel, are they going to use tunnel:part and tunnel:levels?
To set the outline of the tunnel.
Look at
Steps and ramps from one level to the other.
There is not a good sheme for tunnels! Yet.
How to use the same node in a building or a tunnel?
https://openlevelup.net/?l=2#21/50.77934/6.07464
Click in advanced mode on a door or room etc. From here you can import to JOSM.
level=-1-3
repeat_on !!!
This spiral ramp (building) goes to multiple levels in the parking building. In a way this is indoor tagging. Level tagging.
I understand it, but making a set off to draw a way is not correct. (maybe this is tagging for the router, how they implanted level in the shemes, a big question)
I also struggle with this.
JOSM have a nice level layering. 
When a there is a busstop in a road tunnel, it is the same as with railway, should we do it the same?
https://www.openstationmap.org/#17.58/52.5248/13.36987/8.8/55