OP here - thanks for the replies.
Please note that what I was inquiring about was skipping the SECOND leg, not the first. At any rate, at least one of my travel agents agrees with what has been posted by 172, that any no-show results in cancellation of the remainder of the trip.
I do have some experience on this, having forgotten to take a flight on one occasion. I only noticed when I was tidying up my flat and looked at a ticket on the dinning table and saw that day's date was on it. I promptly phoned KLM and asked if I was supposed to be on a flight at that precise instant. "Yes indeed, sir---came the reply---you are listed as a no-show". However, I asked if it'd be possible to reschedule the ticket for the next day and they said no problem, so that leads me to think the cancellation might not happen automatically, as pointed out by FairlieFlyer.