You are correct
You're rolling two 10-sided dice, and for the first die, you need a 9 or higher for success. For the second die, you need a 5 or higher for success. You're allowed to apply a +1 bonus to the result of one of these tests.
You've correctly calculated the probabilities of getting exactly 0, 1, or 2 successes for both scenarios: rolling 1d10v9+1 and rolling 1d10v5+1.
When comparing the two strategies:
For 1d10v9+1:
Probability of exactly 2 successes: 0.18
Probability of exactly 1 success: 0.54
Probability of no successes: 0.28
Total probability: 1
For 1d10v5+1:
Probability of exactly 2 successes: 0.14
Probability of exactly 1 success: 0.62
Probability of no successes: 0.24
Total probability: 1
Your calculations show that both strategies indeed sum up to 1, indicating that you've considered all possible outcomes. So, regardless of which strategy you choose, the total probability of success remains the same.
So, in terms of maximizing your chances of getting a success, it doesn't matter whether you apply the +1 bonus to the first die or the second die. You'll achieve the same overall probability of success....