Hi krass, boolean union will merge faces under certain conditions - if they are both planes that are coplanar or if they both have the same "underlying surface".

So that probably would work on your second case you show but not the first case.

I do want to work on a merging function for the first case you show there but there is not currently anything set up for that.

For your second case instead of merging you could also "untrim" the larger surface to remove the hole:
http://moi3d.com/forum/index.php?webtag=MOI&msg=444.4
http://moi3d.com/forum/index.php?webtag=MOI&msg=9671.5

- Michael