Your synth -- you mean through a MIDI connection? No, the Delta has no MIDI interface. You would need a separate MIDI interface. If you mean through the audio ports, the answer is the same as the first question's -- no, not without submixing. 2 channels in is 2 channels in.
Unless you have something (the synth, maybe?) that has an S/PDIF output. I believe you can use the S/PDIF for another pair of simultaneous inputs if you have a digital source to connect to it.