The suffix or pronoun necessary to turn the verb into a plural (place THEM) is not there so the assumption is that a person wears one crown on his head.
IIRC it was the silver crown. As to the question of the Tznif, the Metzudat David mentions that connection. Zech 3 was a vision of the angels and Joshua as shown in a dream to Zecharia (as per 3:1 and the chapters before which detail the visions shown to Zecharia).
The M"David says explicitly that it is not to be put on Zerubavel but is to be saved for the future king after the final exile is over.
While Zerubavel is associated with the word "shoot/growth" the commentators explain that he is not the final growth referenced here, but from him that final messianic figure will descend in the future.
Since he was told to make crowns and place on the head of Joshua the High Priest, (and it doesn't specify plural or singular) the assumption should be that both would be placed on his head. Especially since he is symbolic of the Branch (the Messiah) who will be both King and Priest. (Which is in line with many other scriptures)
It has almost amazed me the lengths you guys will go to, to avoid having things mean what they mean in the majority of cases. No matter what evidence is presented to you, you find a way to try to nullify it. Doesn't the preponderance of evidence mean anything to you?
v'hayah is commonly translated "and he will be" - yet in several posts you portrayed it as if that is not a common way to translate it. ("and it will be" - "and there will be" are just other possibilities) Your problem here is that if it is "and he will be", that would make the same one sitting on the throne also be a priest on his throne. (and you can't have that)
In Zechariah 6:13 where it said
and he will be a priest on his throne. Suddenly kis'o didn't mean his throne, it just means his chair. Even though you know the King would sit on a throne.
Then there is Ezekiel 21:26-27 where God told them to remove
both the diadem and the crown. He said he would overturn it
until he come whose right it is and I will give it to
him. (Once again it doesn't say them, it says he and him.) It might be a different prophecy, but it is still more evidence of one individual who will have both crowns.
Then in the discussion regarding Psalms 110:1-5
1. The vast majority of times this intro means a Psalm was written by David. (Suddenly you guys don't really know for sure if Psalms 110:1-5 was written by David or form him.)
2. Then you guys say Psalms 110:1-5 is about either David or Abraham or both. So you can choose written by David, if it is about Abraham, and written for David if it is about David, and I'm not sure what you choose when you think it is about both David and Abraham. And I think Tumah acknowledged it could also be about the Messiah.
Who is the individual mentioned in Psalms 110:1? Neither David nor Abraham were Priests, and especially not forever.
3. Then since Psalms 110:4 said YHWH hath sworn and will not repent
you are a priest forever after the order of Melchitzedek. Suddenly chohen doesn't mean priest in that verse, because that would cause you problems. Because even the very name Melchitzedec means King of righteousness. And in Genesis 14:18
Melchitzedec is said to be both King of Salem,
and priest of the most high God. Which would make this individual mentioned in Psalms 110:1-4, both King and Priest. (And there is no way you want the Messiah to be both King and Priest)
4.
Who is the "you" in Psalms 110:4 that will be a priest forever?
And how will that individual be a priest forever? (no matter what you try to choose priest to mean)