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This might not be possible, but does anyone know how to ungroup an STL file in 123D ?

The E3D v6 mount has both the Top and Bottom sections in one STL file.

try using meshmixer, I have had zero success in doing anything with 123D once it renders an STL file. I like using it, but I always save the drawing in the 123D file format so I can edit later, I usually save several times after I make a modification so I have several places to re enter the drawing process.

Tom C

Some slicers will let you split up multi-object files, and you could save the parts afterwards.

In DesignSpark, when you import an STL, if it's separate objects, you can split them after the import.

Okay, I wanted to give it a try but I guess I'll go back to DSM.

You can split it apart in netfabb

123D isn't much more than a viewer for STL files. You can copy and delete the portions you don't want, but it's REALLY REALLY SLOW.
So for this example, make a copy (CTRL-C),(CTRL-V), move it over one direction a few inches/cm so the original and the copy don't overlap, then draw a sketch circle under the portion you want to delete from the first copy and extrude it THROUGH the pieces you don't want. The default behavior is SUBTRACT which will delete anything inside the new cylinder. It's slow, but should work.
As other have said, there are better tools for this operation on STL files!

With meshmixer you simply click Edit (on the side) ---> separate shells.