We're running an almost 100% homebrew system, which uses skills in order to craft objects during play. Allow me to first formulate an example to explain my question better:
Jack is quite the mason and has his Stonecarving at skill level 50. He wants to carve a big stack of stones for his house. He rolls a d100, and must roll equal to or lower than his Stonecarving skill level. This quite obviously gives us a 50% chance of Jack succeeding. No problem here.
Now Jack has also been brushing up his artistic skills and has his Artistry skill also at level 50. Jack wants to carve an artistic statue out of stone. He rolls 2d100, since making the statue would arguably use both the Artistry and the Stonecarving skill. This is where the problem arises.
How do I make this 2d100 roll equal to the d100 Jack rolled earlier for the stone? If I say "you need to roll equal to or below your skill level for both rolls", it gives Jack (theoretically) a 25% chance of succeeding. If I say "you need to roll equal to or below your skill level for one of the rolls", we get a 75% chance of Jack succeeding.
Ultimately, what I am looking for: How do I arrange a 2d100 roll, in order to make it equally fair as a d100 roll?
To further clarify, how do I arrange the dice or what mathematical process should I follow in order to attain the probability of a d100, when rolling 2 d100s? In case of the example, I am looking to have the 50% probability of success, in the paragraph with the two skills being used.
In the case where Jack were to have differing skill levels, I am looking for a success rate based on the average of the skills. 30 Stonecarving and 60 Artistry should then result in a 45% chance of success.