i know all about that, i work in new technologies associated to sound

I just need to use the motu for this and it would be great if skype could work with it... as i need to get the sound from the person talking, the sound of my voice and enter all that in the computer.
My idea was to use motu828 outputs 1&2 that i send to my mixing board in 1 2.
These two tracks are send to two groups and those groups are sent to inputs 3 4 of the motu. My voice is also send to input 3 4 on the motu. What i finaly record is everything coming from inputs 3 4.
You're right, i need a mixer and this is what i'm doing here.
But the outputs of this mixer need to go to an interface to go back to the Mac.
I cannot use the line mini jack of the Mac as an output AND as an input !
Well i could but i would need a double jack for the microphone input + a mixer...
Thanks anyway and happy new year