Sonar will record four separate tracks at once, if you have an audio interface that has four inputs. This one for instance:-
Edirol 6 Channel This would need a firewire port on your laptop.
If you do it your way you can use a simpler and cheaper one like this:-
M Audio 2 Channel This is intended for use with a laptop.
But you will only record the stereo mix, and won't be able to re-balance the mix later.
Hope this helps.
Cheers
Dave