You need about 2 ppm of KH to get a pH of 6, with no CO2 injection. That is 2 ppm, not 2 dKH. You can check this out by using one of the pH/KH/CO2 calculators, such as on Measuring CO2 levels in a Planted Tank. Just remember that the CO2 in the tank will be around 3 ppm or less with no CO2 injection. So, you play around with entering a pH and a KH and seeing what the ppm of CO2 is. When it hits 3 you are in the right ball park.

Practically speaking, you can't get there by blending the high KH tap water you have with RO/DI water.