Would you require a WYE from the WH to the furnace flue?
Only if water heater or furnace has a inducer draft blower on the exhaust.
But “Wye” ?
If that is a TEE connection then the flue is now a horizontal run at that section and violates the required 1/4" per foot rise needed to satisfy code does it not?.
That Tee has a reduction.
Is it 1/4" in 12"?
Looks like it is, not that it really matters.
Unless someone wants to get out their manometer ( what the F*** is that?) and measure the flue draft, I don’t think there is anything to talk about here.
This is going into a chimney, and the draft may be so great that it may require a barometric damper!