Two discs wouldn't be all that horrible, but only if he gets another transformation, and only after he reaches that transformation. The single disc is easy enough to dodge 98% of the time, he could use another one, even if it takes longer to reach a minimum charge. I still believe it would be much more balanced to make the charge level equal the speed boost given to the disc after it's thrown. A fully charged Kienzan should move about twice as fast as a normal Kienzan does now. Nobody charges them up all the way anyhow, it takes too much ki and it doesn't change anything. The more balanced system would make a minimum charge slower than the current disc, rendering it nearly useless.
Anyway, I don't see how his disc is a problem for anyone. Frieza's disc needs to take longer to come back to him, and needs to fly off into space when somebody hits him, even if he's pulling it back. Other than that it's fine, and only the alternate attack on the laser finger needs to be touched up on.
Let's not turn this into a flame war. Mystic Goku has had plenty of those in the past, and I don't think anybody needs to help him start another one.