Yes, both hot and cold must be on for the water to flow through the balancing valve and into the mixing chamber then out of your shower head. This applies to a single handle tempering valve, Moen brand. and not more than 10 or 12 years old.
You see, prior to the new mandatory "anti-scald" feature being introduced to code around 15 years ago, single lever moen shower and tub valves would allow water flow through cold only or vice versa.
So I'm taking for granted that your shower valve is newer than 12-15 years when I answered "Yes" to your original question.....
If your shower valve is older than 12-15 years.....the answer is a resounding, (probably)..."no", both do not have to be on to allow flow through the mixing valve and out of your shower head.
I'm not sure why you would want to run just one or the other through your shower valve instead of both, but I'm sure you have a good reason...so apply the info I have given to your new...or older moen shower valve to get your answer. Good luck and may the Plumb Gods smile upon your courageous soul....
ps: if any other folks with the same question read this forum....it would shorten our time in writing a reply and surely result in a more definite answer to your question if you include the model and/or line name of the plumbing valve in the substance of your written question. pss...thanks
Moen two handle shower no hot water
No but you will get a cross conection problem
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