Physically, you might say that if both legs bear the same amount of weight, they are double weighted. And if both hands are flexed the same amount at the same time or strike at the same time, they are double weighted. If you transfer weight between the legs so that one bears more weight, then you are more mobile, and not double weighted. If you strike with one hand just before the other (even in the posture “push”), then they are not double weighted. Of course, there is the point when transferring weight from one side to the other where both legs or arms “weigh” the same, but it is, just as in the I-ching, a transition point.
So, to use an example from a Taiji form: When you do the opening movement, you do not stand with your feet planted without transferring weight. The weight must be shifted slightly onto the balls of the feet and then back again. That takes care of the front and back of the feet. Weight must also be transferred right to left, so we transfer most of the weight onto the right foot first, then the left, and then back again. The weight transfer then takes on a circular shape: From double weighted to right heel, right toe, left toe, left ball, and back to the center again. (Of course the weight keeps shifting into the next movement.) But the hands must also not be double weighted. So we move the left one first, then the right. The left hand goes yin and begins its ascent, followed shortly by the right hand. Because the left hand moved first and began the transition to yin first, the right hand will turn yin slightly after, keeping them from being the same during the move. As the hand begin their descent, the left one goes yang first, and when the right hand follows it down, it begins changing to yang as well. Although at one moment both hands might be yang or yin, one is more yang or yin than the other, and therefore, even though both might be the same to an observer, they are not in reality.
This same thing applies to the whole body. One side of the abdomen will be yang and the other yin, one scapula will be yang and the other yin, etc. Sometimes the front of the body will be yin and the back yang and vice versa. It gets real subtle and real complex, especially when dealing with your form.
On the more esoteric side, if you meet force with force, you are double weighted. If you react to a situation or a person with anger, you are double weighted. If you throw a punch using just your arm muscles instead of your whole body, you are double weighted. All these are examples of two things working in opposition to each other and hindering each other.
When the unstoppable force meets the immovable object, that is double weighted. So is the force that can’t move anything meeting the obstacle that can’t withstand any force at all.
If you want a definition of yin and yang, that can get pretty complicated, too. For instance, with the first movement of the form, we transfer weight to the right leg, which makes the left yin and the right yang. But if you define yang as giving and yin as receiving, then the left leg is yang and the right yin. If you define yang as attacking and yin as defending, then when we execute the application behind the posture, we first are yin as we defend and then yang as we attack. But if our defense must become our attack, then these blocks are strikes as well, and that is yang, though it is not as yang as the following strikes.
You could also take this movement, lifting the arms to block and lowering them to strike, and look at it as a sine wave. Picture a point on a crest of the sine wave. If the sine wave doesn’t move, but instead this point travels along it, it will be on the crest at the beginning, travel down through the middle, and end up at the trough, and then continue to the crest again. When we perform the block from this posture, the left hand strikes first, in a yin shape, and as it changes to a yang shape to strike, the right hand goes yin to strike block, and as the left hand rebounds off the opponent, the right hand goes yang for the strike. I could go into mind numbingly deep and philosophical detail about the interplay of yin and yang here, and how although the hand might be yin it is really yang, and so on, but this is something you really need to firstly experience and then meditate on, yourself.
These are more everyday definitions of yin and yang and their interplay than you will find in some philosophy book, and they actually have to do with martial arts.
My advice is to not try and define the interplay between yin and yang, but rather try and embody it.
You might ask “how can you be if you don’t know,” and other than trying to refute desCartes’ argument for existence by kicking the table and asking if the table thinks, I would ask you “how can you know if you don’t experience?” Just practice, and the answers will come. And when the answers come, you will know because you can then read the classics with which they have to do and begin to understand a little about what they mean.
And if you get all the answers before I do, let me know, eh? 
“I put forth my power and he was broken.
I withdrew my power and he was ground into fine dust.”
-Aleister Crowley, The Vision and the Voice
