Quote:
when one toilet is flush the other toilets waste does not enter the other toilet, but dis-coloured water does
Your toilets are connected together by a double sanitary tee,(see image) but liquid isn't supposed to jump across the stack into the other bowl. I don't know about a "stench pipe" but the roof vent should be snaked to see ifi back pressure's involved. Put out enough snake not reach the base and 20 feet more. Let us know how it worked out. If that don't get it perhaps the double tee needs to be replaced with a double wye with two street 45's.(see images) to better direct the discharge.