I can't split the audio between speakers and headphones

Hello,
This is my 1st post, I've really like all the help I've seen floating around here. I did a search but all the answers for my problem came up with equipment specific answers. So here I go.
-I have a FirstMix USB DJ Controller PRO
-Cross DJ LE
-A 15" MacBook Pro, their seems to be a regular stock soundcard? It didn't list any names. unless it was "Audio ID 75" or "Intel High Definition Audio"
I don't have a mixer or anything, I'm going:
output RCA->1/8th inch -> Female-to-Female 18th inch -> External computer speakers.
Then Headphones from the front headphone jack.
I'm at a loss. Please help.
This is my 1st post, I've really like all the help I've seen floating around here. I did a search but all the answers for my problem came up with equipment specific answers. So here I go.
-I have a FirstMix USB DJ Controller PRO
-Cross DJ LE
-A 15" MacBook Pro, their seems to be a regular stock soundcard? It didn't list any names. unless it was "Audio ID 75" or "Intel High Definition Audio"
I don't have a mixer or anything, I'm going:
output RCA->1/8th inch -> Female-to-Female 18th inch -> External computer speakers.
Then Headphones from the front headphone jack.
I'm at a loss. Please help.