This TACL command might be what you need:
status *,prog $system.*.emsacoll
That will list all the processes (and their backup processes) that are running the EMS Alternate Collector object file. The list includes the name of each process. You will have to eliminate the backup processes from the list yourself. You can do that visually, by skipping the lines that show the same process name as another line, or you could write a TACL macro or routine to automate that.
This list, plus $0, will show all of the EMS collectors that are running.
I am not sure why you would need to know the names of all the EMS collectors. When there are multiple collectors, it usually is the case that each one was started to collect events from a particular set of processes in the system, so you usually should not send events to a collector unless you already know that collector was set up to receive the kind of events you intend to send it. But I suppose if you are unfamiliar with a system and are trying to find out what has been set up, listing the names of all the collectors could be a useful thing to do.