You wouldn't exactly stick them together to use them, more like use them both at the same time and that would create the steam.
I guess a better way of explaining it is that the crystals hold the magic and use humans have the ability to flow that magic through them and then use it to there will. Hence why they embed it in weapons and all of that since it makes them a more potent weapon. (And also a lot less dangerous since there is some documentation of people mysteriously dying from using raw crystals too much.)
As for the healing, it would simply hasten the process and can help relieve pain. The crystal would most likely have to be in the form of a powder or some type of weird water paste in order for it to be effective and the user would need a secondary crystal to activate the weird paste powder. Again it would only quicken the natural healing process and not like miracle cure everything.
Also yes, bender style for the particular elements. Though using a crystal raw like that would typically drain it faster than, say, if you imbued it into a gun.
And no, I would more say you could send out messages and if another person had a magenta crystal they could possibly return a message. But you can't read minds :)