counting for more than one minyan
If one is in an open area in which more than one minyan is meeting, can he be counted for both minyanim simultaneously?
If, at the kotel, one minyan has started and then someone leaves, and I am there davening with another minyan but I can hear the first, can I count for both? Or if I have finished davening and more minyanim start, can I volunteer myself to more than one at a time to count as a tenth?
I know that according to some opinions, one can be asleep and counted for the minyan, so it isn't that I actively have to answer to anything or even be aware.
Are there any formulated t'shuvot on this?