How do you dilute forward and reverse primers for a master mix?
Would I just add 500μL of my forward primer and 500μL of my reverse
  primer for a mix of 1000μL in which both concentrations are still
  10μM?


No.  Diluting your 10μM solution in half will half the concentration.  Mixing equal parts of 10μM primer will make a master mix where each primer is 5μM

But in general, primers are added to these reactions in great excess, so 5μM of primer might be okay.