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I didn't want to debate it in the thread but I think your response said to connect one yellow to the 502 [com] terminal and the other yellow to the 502 [W] terminal? If that's true I think the 2 yellows are an end switch and when the thermostat calls, 24v is sent to W and the end switch will create a short circuit to the transformer and it will burn out. Let m know how I may be mistaken.
I never saw anyone wire zone valves to be powered by the TT transformner on a relay. I don't think it will power 2 zone valves reliably for long, each zone valve in this case draws about 7va and the on board relays about 2 or 3va.
But, if there were just 1 zone valve, here is a way to wire it using [R] and [W]:
502 [com] terminal - connect to C at Tstat and 1 red wire on valve
502 thermostat [R] terminal - connect to R at Tstat
502 thermostat [W] terminal - connect to W at Tstat and other red on valve
Nothing connects or needs to connect to yellow end switch wires
Thanks for sharing, Dave0December 2020