Preventing pH ping pong
Stretch your pH up and down by adding 50% or more water to your pH up or down. Distilled water recommended. Dilute up to 5 to 1 (water to solution). It will take longer to dose up or down, but you'll be less likley to "ping pong" on the dosing, which uses more solution and causes your PPM to rise.
Is it still possible to dilute the PH up and down after you have already put them in the nutrient reservoir? You would you recommend making a solution before filling the nutrient resevoir?
Dilute away, right in the bottles. To reiterate, this is only for pH solutions. You don't need to worry about how dilute or strong they are relatively. You COULD also dilute nutrient solutions, but you'd have to be very precise on exact relative dilutions. I don't recommend it.