I assume the attached images are close to what you are looking for?
If so, then here's what you can do (this might be a bit of a hack, but I did get the same error "can't join elements" that you are probably getting.
1) Modify the secondary decorative roof sketch and extend it back to the other roof, until from the outside you get the look you want (the roofs join each other).
2) Go the the Home Tab, and in the opening section, select "Vertical".
3) Select the secondary roof to start the sketch. I found it easiest too see the edges in shaded mode (as hidden line mode didn't show the edge where the roofs meet properly yet). In plan, carefully sketch the inverse of the shape you want. The size doesn't really matter, as long as you cut the whole extents of the roof you don't want. Finish the sketch.
That should get you the look that is in my images, which is most likely what you need (minus the wall that would obviously be there). If you are also wanting to cut main roof where they intesect, do the same process as above for the main roof but cut the shape of the secondary roof this time.
I hope this works for you. Let me know if it does so other users know the process works!
Always glad to help!