The city of Bellevue only required a 125 PSI relief on the incoming cold water in the early days. There was no requirement for a T&P on the water heater until they got a new head inspector. I thought it was the dumbest thing in the world and I had my losing argument with the inspector back then. I was building four homes in Bellevue, and did the 125 PSI on the cold and also did the standard T&P on the water heater. There were plenty of homes in Bellevue though during that time that never did get the right solution for the water heater T&P's. Some were not anywhere near an outside wall and draining was going to be a future problem.
I have worked on condos in Seattle that allowed combined reliefs on a stack, but that was a while ago. I would check with your local head inspector and ask what they allow.
My blue pickup truck, 1984 was the year.
NE 6th Street in Bellevue near Crossroads.