What you need to do is send each instrument to a different rewire output channel in Reason (that rack unit at the very top). Odds are you're just getting a L/R out of the mixer ending up in Logic.
For example, if you're using ReDrum, instead of using the L/R Mix out of the mixer into the output, use an output from each channel of of Redrum to the "audio out" rack. You can do this for any of the virtual instruments in Reason.
Once you have them assigned to a channel in Reason, just create a new instrument channel in Logic and assign each track to a ReWire channel corresponding to the instrument in Reason.
Hope this makes sense. It's pretty easy once you figure it out.