Does Solidworks create dimensions for extrusions relative to geometry (rather than using a value)?

I.E. I have a body extruded from one plane to another:

I need to extrude cut the body so I have two square beams on either side of the oval shape in the middle:

If I go into the Equations, Global Variables, and Dimensions window, I cannot use "D1@Boss-Extrude 6" because the dimension is relative to the two planes....

I know there are a few ways to accomplish the same thing, so I don't really need suggestions on how to get what I want (as evidenced by picture 2), but why wouldn't such a feature be possible?