EMB JET CAPT LEARNED THAT HE WOULD HAVE TWO SKY MARSHALS ABOARD; BUT COULD NOT HAVE THE REQUIRED; AND DISCREET. ID CHK AND BRIEFING WITH ONE DUE TO THE PAX BOARDING AT THE SAME TIME.

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DUE TO SECURITY PROCS CAPT MUST CALL DISPATCH ON GND LINE TO HAVE DISPATCH UNLOCK RELEASE TO STATION. DURING THIS EFFORT; ONE OF THE FEDERAL AIR MARSHALS ASSIGNED TO TRAVEL ON THIS FLT PROCEEDED TO THE ACFT. CAPT BECAME AWARE OF MARSHAL UPON RETURNING TO ACFT. MARSHAL INDICATED HE WAS WORKING WITH A PARTNER. HOWEVER; HE STATED THAT THEY SEPARATED TO MAINTAIN THEIR COVER. HE FURTHER STATED THAT THEY DID THIS ON OTHER FLTS. ALTHOUGH I WAS ABLE TO CHK THIS MARSHAL'S ID; I COULD NOT ASCERTAIN THE OTHER MARSHAL'S CREDENTIALS WITHOUT COMPROMISING HIS COVER. THE STATION DID NOT INDICATE THAT THERE WERE MARSHALS WHEN WE ARRIVED. CONSEQUENTLY; THERE WAS NO MANNER TO DISCREETLY CHK WITH THE OTHER MARSHAL SINCE THE STATION BEGAN BOARDING AS SOON AS I RETURNED TO THE ACFT. IF THE PROTOCOL STATED BY THE LEAD MARSHAL IS VALID; THEN THE PROCEDURE TO CONFIRM ID'S NEEDS TO BE ADDRESSED IN OUR FLT MANUAL. IF THIS PROC IS NOT VALID; THEN THE AIRLINE NEEDS TO ADVISE THE MARSHAL'S SERVICE THAT THIS PROTOCOL IS NOT CORRECT. BETTER COMMUNICATIONS FROM THE STATION WOULD ALSO HELP. THE STATION NEEDS TO CHK WITH THE CAPT BEFORE THEY BEGIN BOARDING TO DETERMINE IF THE CREW AND THE MARSHALS HAVE COMPLETED THEIR BRIEFINGS.