Hi Steve,
You have to select the grey piece and the red piece before hitting CTRL-T, then select the grey bit you want gone, then the red bit you want gone, then hit return to finish. Think of this as the Mutually Assured Destruction variant of Trim!
Trying to select just one piece and trimming the other, followed by selecting the second to trim the first does not work because the second trim fails. (Although this can be rescued by taking the intersection between the truncated piece and the untrimmed piece and using that to trim the latter.)
When you come to do the other end, know that, in Rhino 5 at least, there will be some positions where even the MAD trim won’t work because Rhino can’t solve the intersection. If you are unlucky enough to hit that problem you will need to nudge the end piece along the axis a little and/or adjust the slope of the shoulder slightly to get a working intersection.
HTH
Jeremy