Controlling external MIDI hardware with Sonar X2

You could connect the synths to a single pair of MIDI in/out sockets by using the MIDI THRU on each synth to feed the next in line. Sometomes known as a "daisy chain".

Each synth would need to be using a different MIDI channel. You then create the necessary MIDI tracks in Sonar, point them at the interface's MIDI ports and set each channel to only send the MIDI channel ised by the relevant synth.

I prefer to use a MIDI interface with more ports on it so each synth uses a single port because every additional synth in a MIDI "daisy chain" sees the MIDI very slightly delayed compared to the one before it. Not a problem with a couple of synths usually but can be with several.

If any of the synths have USB sockets for MIDI then that's another possibility and can be the simplest way of connecting MIDI hardware there is as each synth will have its won dedicated MIDI ports in Sonar.