I guess the effect was simple: when 2 plug-ins get the same signal ("Play"), both try to "toggle" the state, if it was "Stop" it starts play, if it was "Play" it pause.
So... if the second plug-in is called right after the first one, it can "see" that transport is still in "Stop" and switch it to "Play". It kind of work then. But as soon as both plug-ins are called with some delay, the second see the transport in "Play" (triggered by the first plug-in) and "Pause" it.
riojazz is right, Sonar supports many surfaces at the same time. But hooking one physical surface to more then one surface plug-in is looking for troubles (still in some situations that was used).
Note that several Mackie (compatible) units will work as one with stock plug-in, the plug-in supports only "one set" of Mackie units (XTs, C4).