Neither approach is correct. With 5 mg of each in 1 mL total, 0.2 mL is one-fifth of the vial and therefore contains 1 mg of X and 1 mg of Y. To reach only 0.2 mg of each you need to draw 0.04 mL.
This question shows up on reddit and here from time to time. The way some people explain it seems off to me, and since I'm getting ready to order a couple blends I want to be sure I have it right. How is blended dosing actually worked out? My first guess was that 5 mg of X plus 5 mg of Y in 1 mL would be 5 mg of the mix per mL, so 0.4 mL would give 200 mcg of each. What I see others saying is that the same vial is 10 mg per mL, so 0.2 mL would be the 200 mcg dose, but that would only deliver 100 mcg of each. I'm checking because my version gives the full 200 mcg of both in 0.4 mL while the other version appears to give only half that amount. Am I misunderstanding how people are doing the math?