BOTH B737 FLC MEMBERS ARE AWAY FROM THE FLT CTLS WHILE ACFT IS AIRBORNE.

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DURING CRUISE FLT; BOTH CAPT AND I DECIDED WE NEEDED TO USE THE LAVATORY. THE CAPT CALLED THE FLT ATTENDANTS TO SET UP THE PLAN. 1 FLT ATTENDANT CALLED THE COCKPIT AND SAID THE AREA WAS CLR AND GAVE US THE CODE WORD. CAPT GOT UP; UNLOCKED THE BAR; AND LEFT THE COCKPIT WHILE THE FLT ATTENDANT REMAINED IN THE COCKPIT. THE FLT ATTENDANT SAT ON THE SIDE OF THE EMPTY CAPT'S SEAT. CAPT THEN CALLED THE COCKPIT AND GAVE ME THE CORRECT CODE WORD. I TOLD THE FLT ATTENDANT THAT HE WAS READY TO COME IN. TO MY SURPRISE; SHE DIDN'T MOVE. I LOOKED AT HER AND ASKED HER IF SHE WAS GOING TO GET UP. SHE SAID SHE WOULD; BUT SHE NEVER DID. DUE TO THE FACT THAT I REALLY HAD TO USE THE BATHROOM AND THE FLT ATTENDANT'S RELUCTANCE TO GET UP AND OPEN THE DOOR; I GOT UP AND OPENED THE DOOR. THE CAPT CAME IN AND I WENT TO THE BATHROOM. THE FLT ATTENDANT DID GET UP AND MOVE WHEN THE CAPT CAME IN. THERE WAS A MOMENT WHERE NO PLT WAS AT THE CTLS. THE SPEAKER WAS ON; SO WE COULD HEAR ATC THE ENTIRE TIME; BUT THE LACK OF SOMEONE PHYSICALLY AT THE CTLS WAS A MISTAKE ON MY PART. I SHOULD HAVE INSTRUCTED THE FLT ATTENDANT TO GET UP AND OPERATE THE DOOR. I WAS IMPATIENT WITH THE SIT. I BELIEVE THE FLT ATTENDANT WAS CONFUSED SINCE BOTH PLTS WERE GOING TO USE THE LAVATORY AT THE SAME TIME. USUALLY IT'S JUST ONE.