Plumbing – Toilet gurgling when another one is flushing: 6-way joint

I have 2 adjacent bathrooms, with toilets opposite of each other. I recently upgraded one older toiled that took a minute to flush with 3.5 gallons of water to 1.2 GPF model with "aquapiston" marketing blurb. All nice and dandy, except this piston started shooting stuff up another toilet's pipe on the other side of the wall, causing it to gurgle.

This is what it looks like from below:

So the obvious reason is that vent pipe is one of the 6 pipes connected here (the house was built in 1960-s). Toilet 2 shoots stuff through the pipe directly into Toilet 1 drain. This blocks the vent, and causes Toilet 1 to gurgle until stuff starts flowing AWAY.

So, what creative solution can I apply in my situation to eliminate the problem, without completely redoing the junction? Preferably one that does not require custom fabrication of fittings 🙂

EDIT: White PVC pipe is a vent pipe for a sink that drains into a basement septic holding tank (not vented outdoors… Not sure why)

EDIT 2: One solution I considered was adding a Tee to the downpipe from Toilet 1, and connecting it to the vent above the 6-way connection. But space constraint makes it hard to add it, plus it looks like it'll cost me $100 in parts to add those 2 Tees. Is there a more economical way to add, say, 1" pipe connecting those? Would it fix the issue?