B737 FLC TAKE ACTION DUE TO A TCASII RA FROM KNOWN AND A SECOND UNKNOWN ACFT PASSING IN THEIR VICINITY WHEN EXECUTING THE OVETA SID FROM LAS.

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ON DEP OUT OF LAS; DEP GAVE US 7000 FT ON THE OVETA 3. WE WERE TO MAINTAIN 7000 FT FOR TFC BEHIND US WHICH WOULD BE A CONFLICT ONCE WE TURNED TOWARDS THE BOULDER CITY VOR. ABOUT 1/2 WAY THROUGH THE TURN; WE NOTICED ANOTHER TCASII ACFT BELOW US (500 FT ON TCASII). WE WERE LEVEL AT 7000 FT TRYING TO GET A VISUAL ON BOTH THE LOWER TFC (BY DEP) AT 2 O'CLOCK POS. WHILE STILL IN THE TURN THROUGH N; WE GOT AN RA TO CLB. I (THE FO) INITIATED A CLB UP TO 7300 FT TO AVOID THE LOWER CONFLICT; BUT DIDN'T WANT TO GO HIGHER UNTIL I COULD SEE THE TFC AT 7500 FT. AT 7300 FT I PICKED UP THE 7500 FT VFR TFC AND THE RA WAS RESOLVED. I DSNDED BACK TO 7000 FT ONCE THE LOWER TFC WENT BEHIND US. THE CESSNA HAD STARTED A 360 DEG TURN TO AVOID THE AREA AND WE CONTINUED OUT ON A 080 DEG HDG TO RESUME THE DEP. I DON'T BELIEVE DEP EVER HAD THE LOWER ALT TFC ON RADAR; SINCE THEY NEVER CALLED THEM OUT. WHEN THE CAPT ADVISED THEM WE WERE CLBING; THEY WANTED US TO DSND BECAUSE OF THE OTHER HIGHER. WE NEVER SAW THE LOWER ACFT; WE THINK WE PASSED WITHIN 2 MI AND 300 FT OF IT. THE HIGHER ACFT WE PASSED 2 MI AND 500 FT. THE ONLY SOLUTION WE SAW WOULD'VE BEEN FOR US TO ROLL OUT ON A NNW HDG TO AVOID THE AREA ALTOGETHER. HOWEVER; I DON'T KNOW IF THAT WOULD'VE HELPED WITH THE ATC OR THE UNKNOWN TFC SINCE WE NEVER SAW IT TO GET A GOOD FEEL FOR ITS HDG. SUPPLEMENTAL INFO FROM ACN 523446: WE WERE LAYERED BTWN 2 VFR GA ACFT. BOTH WERE BEING TRACKED BY DEP. THE FACT THAT DEP HAD NOT CALLED THE 6500 FT TFC PRIOR TO THE RA DID NOT GIVE ME MUCH TIME TO SCAN BEFORE RESPONDING TO THE RA. WE WERE BOTH AWARE OF THE 6500 FT (CLBING INDICATED ON TCASII) AND 7500 FT (LEVEL INDICATED) TFC. IT WAS AN UNCOMFORTABLE BUT MANAGEABLE SIT. DEP WAS VERY NEUTRAL; NEITHER HELPFUL NOR HURTFUL; IN THE SIT. THE TURN BY THE 7500 FT TFC; WHEN HE HAD US IN SIGHT; AIDED IN MITIGATING THE SIT.