Hi Chiel,
It's not possible to combine more than one depth in a single boundary, but there is a way to achieve what you want by using extra boundaries configured as "islands". It's a technique I use myself for situations exactly the same as yours, where you want a boundary that does not influence the depth, but the technique would also work if you had a boundary that required a different depth (e.g. a dam wall).
The idea is that you create an artificial "island" that acts as a barrier. If you want this to be a simple barrier, and not contribute any depth data at all to the project, then configure it so that
Provide Depth and
Show in Map are un-checked. Draw the island with the barrier where you want it, and a few points away from the main map so that the island forms a polygon.
The "island" is selected in this image:
zoomed in:
and the finished map - note that the fake island does not contribute any depth data, and is not shown in the finished map:
It's also worth noting that you can get exactly the same effect by limiting the map region in the
define map view, but there are times when this can't be done.