If you are using a 25 ml sample size then you should use two scoops. If you are using a 10 ml sample size, then two scoops will give a darker more clear endpoint, but usually one scoop is sufficient and will give you nearly the same result. You can experiment using a 10 ml sample size both ways (one vs. two scoop) to see the difference in readings. The purpose of the powder is as an indicator dye and since you are just looking for a transition from colored (pink/red) to clear, the amount of dye isn't critical. You just need enough for the "last drop" transition to be reasonably distinct.
As an extreme example, if you were to use a fraction of a scoop of powder, then the sample would be so faint, especially as you were getting towards the end of the test, that you would have a hard time seeing it go from pink to clear and you'd probably underestimate the chlorine level. At the other extreme with excess powder, the transition would be easy to see but you'd use more powder and it's possible that there might be some residual faint pink remaining as visible (this last effect is speculation on my part and may not occur).
Richard