Most soft synths will respond to any MIDI channel by default, so it's generally not necessary to make channel assignments, but since SONAR cannot send the same MIDI stream to two ports/synths from one track, it's necessary to have two MIDI tracks echoing the same input to the two synths. The easiest way to set this up is just to insert two instances of the synth as Simple Instrument tracks, set their inputs to your keyboard/controller, and enable Input Echo on both tracks.
This addresses "playing" two synths live simultaneously from the same controller. In order to have the two synths play the same recorded MIDI part, you would need to either record the same part to both tracks, or record on one and copy it to the other. The advantage of the second option is that you can make a linked copy of the clip so that any edits to the MIDI you make later only need to be done in one track.
Being able to drive two synths from a single MIDI track is a long-standing open feature request. There are workarounds involving third party virtual MIDI cables that you might want to look into if you're going to be doing a lot of this.