The game can distinguish between them, even if they both work for the lane they are in. For fills and freestyle mode, the cymbals sound like cymbals and the drums sound like toms.
I believe that they emulate the same button as the pads, but also send another button, maybe R1 (just an example, I don't know which button it is) or something. So, hitting the green pad sends the X button, but hitting the green cymbal sends X and R1 at the same time. This allows the Rock Band 2 set with cymbals to work with the Rock Band 1 game. Rock Band 1 doesn't know what to do with the extra button, so it just ignores it, but still activates the "lane".
I was hoping someone would have the cymbals, so we could firgure out what the second note is. I just found an explanation of how they wired the cymbals, so with a homemade adapter, we could probably use a normal e-cymbal.
http://www.rockband.com/forums/showthread.php?t=86886It uses a 1/8 trs connection wired very strangely. I think it is the tip is unused? 3.3v, the ring is ground, and the sleave is the signal. There may be an inverted op amp in there too, the people on the rockband.com forums couldn't seem to make up their minds. I tried it with my GHWT cymbal and it triggered fairly randomly and the wrong notes. When I had it plugged into the green jack, I got green and red at the same time (not through crosstalk), yellow, blue, yellow and green at the same time, and, rarely, just green by itself. It seems like the softer hits hit more often than harder ones. it Apparently, the 3.3v tip is needed somewhere.