Oh yea right, Now I get the Ying-yang thingy. While I've to admit, sometimes this rules are a little confusing I really need to study them properly. Besides I really don't like asking question can of a habit.
But here's a question though, What of KKG such as Lava release which comprises of Earth and Fire element? Aren't they also Yin-yang technique?
The more questions you have, the better. Don't hesitate to ask questions.
Good question.
See, for a KKG, you're not using Earth and Fire Releases at "once". What you're doing is, that you're gathering your Earth and Fire chakras and forming a new advanced chakra nature itself. Whereas, using a Earth-Fire combination is different, like for eg: a earth-fire combo, forming molten earth, almost Lava, but not lava itself.
Basically, every jutsu requires yin or yang, since Genjutsu is also technically Yin Release. So is Nara clan's Shadows. but Y/Y(Yin-Yang) in our RP, the custom Y/Y i.e. states that unless you get trained in it, you can't use two elemental jutsus at once.
It's different forming a earth-fire combo, and it's different forming a new advanced chakra nature, like Lava, altogether.
For one, you have "Lava" natured chakra, and for the other, you have Earth and Fire natured chakras used simultaneously. That is, you can use an Earth-Fire combo, but it takes two jutsu slots, i.e. you can form a loose slab of earth, and then after you finish forming the earth jutsu, you can melt it by using a fire jutsu.
Let's take a simple example:
Do you think you can form Wood Release by simply using Water and Earth simultaneously?
No. You can't. It would be different using a Earth-water combination to turn the earth wet or sticky or w/e, and a different thing to form an advanced chakra nature, like Wood.
TL;DR:
A KKG involves channeling two chakra natures, and forming a totally new and advanced chakra nature. Eg: Scorch Release(Wind and Fire)
Yin Yang in our RP gives you the ability to use two elemental natures at once(i.e. for eg a Wind and Fire jutsu). Logic states that you have to fuel chakra to your stream jutsu(be it wind or fire), and when you exhaust your chakra for the technique/you stop the technique, then only can you move on to another technique. Yin Yang lets the user bypass that. You can use two stream jutsus at once, i.e. wind and fire streams together to form a deadly combo. But, the combo wouldn't form Scorch, lol.
KKG means bloodline limit, something that is passed by blood relations. That's why Kakashi couldn't copy KKG's.