Water Chemistry Alteration.

  all.

So, as part of my current project of a (kinda) low tech planted tank, I am planning on keeping nice, peaceful, shoals of small fish including tetras. However, my tap water is basically liquid rock, carbonate hardness is at 10, general hardness 14.
I am now producing RO so, the plan is to reduce the hardness but not lose all of the trace minerals etc by, mixing RO with treated tap water. so....

Question 1, will this work as I think it should?

Question 2, if I'm mixing the two 50/50, am I reducing the hardness by half? Is it that simple?

Thoughts and ideas welcomed