First off, the single chuff is generated by the RS board when you crack the throttle, that wasn't coming from the switch.
There was a way to inject the chuff from a switch into the TAS boards, Mike Reagan would know. You can also inject chuff into the R2LC pin-17 directly, grounding that pin will generate the chuff.
Note also there are multiple versions of the TAS EOB boards, and I'm sure there's lots of wrinkles that few know about all of them.