I considered adding a second sensor, but ended up going the easy route first and adding a bunch more strobes. So far that is working for me. With strobes spread around the machine area, I've notice that I tend to notice the reflection of a strobe on the machine even if I'm not glancing up to look directly at a strobe. One of my strobes has a magnetic base (the kind you see the TV cops sticking to the top of the car for chase scenes). With a long-ish cable to it, I can stick it to whatever machine I'm running so it is impossible to miss. But as a practical matter, the only large volume chip producer I run for long periods is the jointer/planer, so the magnetic base light so far just sits by that.
If I was going to add a second sensor with a delay to shut down the collector, I'd be inclined to use a split emitter/detector sensor across the bottom end of the flex hose between the drum and the cyclone. You'd want a fairly short delay, just long enough to avoid false trips from individual chips blocking the beam path, because once your bin is full to that point, you don't have much time to stop if you're planing.
--I had my patience tested. I'm negative--