Mac Config. Audio Problem (Cross LE & U46MKII)VCI-100 MV

I just bought a VCI-100 MV edition and I'm having a problem configuring the preview and output functions with a U46MKII soundcard. I can't preview a song while playing another over the air. I hear music playing on my speakers and headphones (plugged into the U46MKII) at the same time. I can also configure music to play only on my headphones when I press the headphone button on my VCI-100 but I can't play out on one deck and preview on the other at the same time. I've tried every configuration available on the Cross LE audio preferences. I have my RCA's plugged into the 1-2 output of the U46MKII going into the line in of my computer. (I've also tried this using only the master output) I'm using an iMac 24" at the moment for test & home use but I'll be DJ'ing with a new 2.0 Unibody Macbook. I've been working gigs with a VCI-300 and ITCH with out a hitch (pardon the pun)on both my iMac and macbook. The VCI-300 has it's own soundcard so when I plug my RCA's from the 300 into my computer line-in and use my computer headphone jack for output(external speaker system plugged in to the headphone jack), there's no problem. I can preview on one deck while the other is playing over the air. If I use the same computer configuration for my VCI-100, it should work if I have the U46MKII and Cross LE software configured correctly, therefore, it can't be a configuration problem with my computer but must be with the U46MKII or the audio preferences of the Cross LE. Can anyone help with this problem. I can't mix unless I can preview the other deck.