Good questions and good points. From my limited knowledge,
NOTHING is pure yin or pure yang. All things have elements of the other. However, many things, tend to be
MOSTLY either yin or yang. As far as I know, nothing is purely "balanced" either, as this is a dynamic model, not a static one.
Again, from my limited understanding, in order for a
SYSTEM to be balanced, certain parts of the system tend towards either yin or yang.
Yes, you can pick up energy anywhere on the spectrum from either hand. And the hands tend to be mostly one or the other. The left side tends towards yin. The right side tends toward yang.
Where we often get bogged down using our "western rational minds", is to a) go beyond the idea of this as a model and b) to understand it as static.
To truly understand the "TAI CHI"(Supreme Ultimate...i.e. the manifested universe) symbol, (that is the symbol that many people call "yin-yang"), you have to see the little dot of white in the black, and the little dot of black in the white, as well as the shape of the blackness and the whiteness as impying movement and change.
However, even thatidea cannot be an absolute. If everything was changing so much so that it was its opposite, than it would not be able to exist. The nature of cold is yin. Cold cannot be total yang. Yet it has a seed of yang in it.
Take water for example. Water is liquid, which is yin. But water can wear away rock. Rocks are yang, but wearing away something is also yang. Is water yang? Well, yes and no!
If you add more yin to water, by making it very cold, i.e. freezing, now water is ice. Ice is hard, therefore yang in that way. But it is also cold, so it has that yin element. It is also brittle.
If you add heat to water, it now become steam, which is even more yin in consistency than liquid, but it is hot, so it is yang.
The left hand is yin in the map. For focus and consistency, it is best to stay with that model, because this is talking about the energy of the entire body, NOT just the hands.
So, part of the answer to your question is that the hands do get more balanced. But if they changed completely, that would not be balanced. And if there was no difference between them, that would not be balanced.
However, there is something to what you say, not in terms of changing the yin/yang nature of each hand, but for the entire system. WhatI mean is, the more you practice using both hands as much as possible, you do bring balance to your body and mind. I.e. try using your non-dominant hand for things you use your dominant hand for, e.g. writing, or lifting a cup or opening a door. I do believe this can bring more yin/yang balance to the entire system.
It is one of the reasons why in many Tai Chi Chuan (Supreme Ultimate Boxing) schools, we always try to eventually learn and practice all of the forms (katas-- for those familiar with that terminlogy from other schools and another language

) on
BOTH SIDES. In other words, everything we learn in one direction, we eventually practice it from the other direction as well. One way to do this, is simply to practice a "mirror" of the form--each movement is reversed. If we punch with the left hand in one part of the kata, when we do it "left style" (as we sometime refer to the reverse form), we will punch with the right hand there.
Another way, which one of my teachers taught, was to make sure that every movement in the form has its opposite built right in, so you only have to do one form. It adds a lot of movements and time to the form, but it is an interesting and effective way to deal with the very issue thatyour question addresses.
I hope that helps.
If not, how about this:
Here's your original questions:
The right hand is yang energy, the left hand is yin. After much practice, or some practice, would they be balanced? Or will they always be yang and yin, seperated?
And following the Tao, the possible answers could be:
Yes, and yes
No and no
Yes and no
No and yes
and then just "maybe"
So, if none of my above answer works for you, I would just take my pick of one of these five options or all of them.
Or to put it another way, "the map is not the territory".Alfred Korzybski