ACR LEFT ASSIGNED ALT; SETTING UP POTENTIAL CONFLICT WITH ANOTHER IFR ACFT. PLT DEVIATION.

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WE WERE INBND TO DTW IN MLG X WITH THE F/O FLYING AT ASSIGNED ALT OF 7000' AT 250 KTS. I CHANGED MY RADIO TRANSMITTER TO THE OTHER VHF RADIO TO CALL THE COMPANY THAT THE FLT WAS IN RANGE. DURING THIS PROC; I TOLD THE F/O I WAS OFF ATC FREQ. HE THEN KNOWS THAT HE WOULD ANSWER ATC WHILE I TALKED TO COMPANY. MY AUDIO WAS INTERPHONE (EARPHONE; AUDIO SPEAKER OFF) SO I COULD HEAR THE COMPANY AND NOT ATC. HOWEVER; THE F/O'S OVERHEAD SPEAKER WAS ON. AS I FINISHED TALKING TO MY COMPANY; I HEARD 'OUR CALL SIGN...6000'.' I REACHED OVER AND TURNED OUR ALT WINDOW/REMINDER FROM 7000 TO 6000'. I SAW THE F/O PICK UP HIS MIC; BUT DIDN'T HEAR HIS RESPONSE BECAUSE OF THE PREVIOUSLY EXPLAINED CONFIGN. I THEN COMPLETED THE CONVERSATION WITH COMPANY AND SWITCHED BACK TO ATC RADIO CONFIGN. DURING THAT TIME; THE F/O DSNDED TO 6000'. WHILE LEVEL AT 6000'; ATC QUERIED OUR ALT. I SAID WE WERE AT 6000'. HE THEN INFORMED ME THAT THE TFC HE CALLED OUT PREVIOUSLY WAS AT 6000'! BUT; BY THIS TIME; THE TFC HAD BEEN WELL CLR AND HE SAID TO STAY AT 6000'. I APOLOGIZED. WHAT HAPPENED WAS THE CTLR CALLED OUT THE TFC AT 6000'; I HEARD THE ALT AND THOUGHT I WAS SETTING THE ASSIGNED ALT (OVER THE F/O'S CABIN SPEAKER). THIS OBVIOUSLY SET THE F/O IN CONFUSION AS HE ANSWERED TO ATC CONCERNING THE TFC. BUT; HE SAW ME (THE CAPT) REACH OVER AND SET 6000 FROM 7000'. HEY! THE CAPT'S ALWAYS RIGHT; RIGHT? WHOA; WRONG! DOWN HE GOES TO 6000'. HOW MANY TIMES DOES THE RIGHT AMOUNT OF CIRCUMSTANCES OCCUR WHEN YOU ARE BUSY? THIS MLG TYPE IS THE BUSIEST COCKPIT IN OUR FLEET.