Hi Lelio,
It is probably best to have one synth completely control the other rather than trying to sync them in this way if you want to use them live. I find that sync-ing sequencers can give you headaches.
Okay, so you can use one sequencer only. I don't know about the SY-77 but I know that the Triton can have its sequencer tracks set to play internal sounds, external sounds or both. Consult the manual for what each of these means. So, for any tracks that you want to play back just SY-77 sounds, you would set them to external and then it would not play any internal Triton sounds.
You would need to connect a MIDI cable from the MIDI out of the Triton to the MIDI in of the SY-77 in order for this to work. You would not have the ability to play a sound on the SY-77 (from it's keyboard) along with a Triton sound though since the Triton would be the master so-to-speak. You could, however, play a sound on the Triton and have a sound also play on the SY-77.
When you switch to program or combi mode, you would have to be careful that the Global MIDI channels for each of the synths was on differnet MIDI channels. Otherwise, every time you play a sound on the Triton, it would play back a sound on the SY.
HTH,
Fernando