B735 DEPARTING SNA CLBS TO ATC ASSIGNED ALT WHICH RESULTS IN LOSS OF SEPARATION.

Narrative

I WAS THE PF ON FLT TO PHX. THIS WAS MY 4TH LEG ON THE SECOND DAY OF A 3-DAY TRIP. THE ORIGINAL FO WAS REPLACED BY RESERVE FO'S. THE FO ON THIS LEG WAS THE 3RD FO IN 2 DAYS; THE 2ND THIS DAY. HE IS A NEW HIRE FROM A CLASS 5 MONTHS AGO AND STILL ON PROBATION. HIS PERFORMANCE WAS GOOD AS PER COMPANY SOP. THE FLT WAS NORMAL; EXCEPT WE STEP CLBED 1000 FT OR 2000 FT AT A TIME UP TO FL180. LEVEL AT FL180; WE RECEIVED THE CLRNC 'CLRED DIRECT TRM; CLB TO FL220.' I HEARD AND ACKNOWLEDGED FL220. THE FO DIALED FL220 IN THE ALT ALERTER AND I HEARD THE FO READ BACK THE CLRNC; 'DIRECT TRM CLB TO FL220.' I INITIATED A CLB USING VERT SPD MADE WITH THE FLT DIRECTOR AND AUTOPLT. PASSING THROUGH FL213; CTR SAID; 'MAINTAIN FL210.' I DISENGAGED VERT SPD MODE AND PUSHED THE NOSE OVER TO DSND TO FL210. REACHING FL210; I ENGAGED ALT HOLD. I DON'T REMEMBER IF I RESET THE ALT ALERTER TO FL210. I THINK I DID. CTR POINTED OUT A SLIGHTLY SLOWER A320 ABOUT 11 O'CLOCK POS; AND 3 MI AT FL220; SLOWLY CLBING. I HAD CHKED THE VSI AS WE DSNDED TO FL210 AND SAW A SOLID WHITE DIAMOND 600 FT ABOVE US AT AROUND 2 1/2-3 MI. NO AURAL WARNING WAS HEARD. I ASKED THE CTLR IF WE HAD MISTAKEN OUR CLRNC AND HE REPLIED 'I DON'T KNOW.' AFTER A LATER CLB CLRNC AND HDOF; THE NEXT SECTOR CTLR ADVISED US TO CALL THE CTR ABOUT A POSSIBLE ALTDEV.