For one liter solution, add your NaCl, KCl to 800 mL water and dissolve. Put in a pH electrode and add 0.01 M NaHCO2 until the pH is 6.5. Make to one liter and touch up the pH with more NaHCO3 if needed. Seal the solution from the air.
In this range, theoretical calculations will be useless. You're at the first pKa of carbonic acid, so the amount you add will depend on how much CO2 is dissolved in the solution.
Dangerous Bill