NMAC BTWN XING TFC DURING RADAR VECTORING FOR AN IAP ILS APCH.

Narrative

WHILE BEING VECTORED FOR ILS RWY 36L AT CLT; WE WERE ADVISED OF THE OTHER ACFT W OF OUR POS THAT WAS BEING VECTORED FOR THE ILS RWY 36R. HE WAS AT 3600 FT AND WE WERE CLRED TO 4600 FT. FO WAS FLYING THE ACFT ON AUTOPLT. AT APPROX +/-4700 FT; I DIVERTED MY ATTN TO THE TCASII. I QUERIED THE FO THAT WE WERE GOING TO THE L SIDE AND THE OTHER ACFT WAS GOING TO THE R SIDE. HE RESPONDED WITH AN AFFIRMATIVE. I WATCHED THE TCASII AS OUR FLT PATHS GREW CLOSER. UNAWARE; THE FO CONTINUED THE DSCNT THROUGH 4600 FT. I CONTINUED TO FOCUS ON THE TCASII. WE RECEIVED A TA AND I RESPONDED THAT THIS DOES NOT LOOK RIGHT. AT THAT TIME APCH CTL CALLED TO CONFIRM OUR ALT AT 4600 FT. REALIZING OUR ERROR (I SAW US DOWN TO 4200 FT) WE IMMEDIATELY CLBED BACK UP TO 4600 FT. DURING THE CORRECTION WE RECEIVED AN RA TO 'CLB.' WE LEVELED OFF AT 4600 FT AND COMPLETED THE FLT WITHOUT FURTHER INCIDENT. SOME CONTRIBUTING FACTORS: 1) ONE PLT MUST FLY THE ACFT AT ALL TIMES WHILE THE PNF MUST CONTINUE TO MONITOR ALL ASPECTS OF THE FLT. 2) I THINK XING THE APCH PATHS OF 2 DIFFERENT ACFT IN IMC DIVERTED OUR ATTN TO THE TCASII. 3) EVEN UNAWARE OF THE ALTDEV I SHOULD HAVE REACTED QUICKER TO WHAT I WAS SEEING ON THE TCASII. IN THE FUTURE I WILL BE CAREFUL NOT TO CHANNELIZE MY ATTN ON 1 AREA (TCASII) AND MONITOR THE FLT. SUPPLEMENTAL INFO FROM ACN 359627: RA SOUNDED AS RECOVERY WAS INITIATED. DURING THE CONFLICT WE WERE IFR AND NEVER SAW OTHER ACFT. REVIEWING CONFLICT I CANNOT BELIEVE I BROKE RULE #1: FLY THE AIRPLANE! I ALLOWED MYSELF TO BE DISTR. THIS IS ESPECIALLY FRUSTRATING TO ME AS I HAD BRIEFED THE UNUSUAL PATTERN WE WOULD BE DOING (R BASE FOR RWY 36L) AND THE POTENTIAL FOR CONFLICT WITH POSSIBLE TFC ON RWY 36R.