This sort of cut is why I have two track saw setting jigs. I would use the one for cutting a piece ahead of the track so the track can be on the wide piece of plywood and the cut piece ahead of the track. Works great. Simple and cheap to make. This is also a cut that Festool offers a special setup for. It also allows you to keep the track on the wide piece of wood.
My jig has a dado that goes over the rib that guides the saw and a movable block with a pointer and stick on scale. For this jig, the distance accounts for the saw kerf and sets the track back the distance you want to cut plus the kerf. The other jig goes the other direction working like your parallel guides. I could have made two but since I clamp, I just use the same jig on both ends clamping as I go (and double checking the end I do first).
I cut on narrow pieces of wood sometimes by supporting the track with scrap as has been suggested. But I can't clamp under those circumstances and I like to clamp so I avoid it.