Here is a quick snip from their FAQ:
Can CADWorx Structure be ran concurrently with and inside a CADWorx Plant session?

Yes. CADWorx Structure can either be run concurrently alongside an active CADWorx Plant session or it can be initialized via a manual AutoCAD command-line function from within CADWorx Plant. In each case, CADWorx Structure is a separately running application from CADWorx Plant, which results in the use of one license per application running at any time. Refer to Appendix B of the CADWorx Plant 2017 User Guide for more information on how to load CADWorx Structure from within CADWorx Plant.

More information can be found here:
http://icas.intergraph.com/cwx-structure-licensing

I think its nice having a tool that is all inclusive, but as dgorsman stated, its not all that common to need to run both at the same time.