In many cases, for published adventures and challenges, D&D will use multiple DCs. This is especially common for knowledge or perception checks.
For example, with a DC 10 knowledge check you might know that trolls regenerate damage; with a DC 15 you might know that fire prevents said regeneration.
When designing challenges as a GM, you could extend this to other challenges. Make opening a lock at the cost of breaking your tools DC 18, while opening the lock without negative consequence would be DC 22.
Average The Skills
If he has to use two skills, average the two skills together and then make one roll. In this case, that'd be a single roll to get 50 or below, since he has 50 in both skills (so the average is 50).
If he was better at one skill than another, it'd look slightly different. Say he has a 50 in Stonecarving and 25 in Artistry. That makes the average of them 37.5, so he'd have to get a 37 or below (or a 38, depending on how you want to round).
That basically treats it like he's using both skills and has to succeed on using them in combination, rather than having to succeed on separate rolls for both. It also keeps it to a single roll with similar odds, and is relatively simple to implement for players.
Alternative - Geometric Mean
The downside to averages is that if you're really good at one skill (say 100 in Stonecarving) and really bad at the other (0 in Artistry), you still have a 50 in the combined skill. That might not be what you had in mind, as someone with no artistic talent doesn't suddenly gain it just because they are working with stone.
In this case, an alternative method is to take the Geometric Mean. For two skills, that is this formula:
$$\sqrt{skill_1 \cdot skill_2}$$
So, if you have 100 in Stonecarving and 0 in Artistry, you do \$100 \cdot 0\$, which is 0. The square root of that is 0. As a result, you now need to at least have 1 skill point in Artistry in order to attempt the combined result. If you did have Artistry 1, you'd get \$100 \cdot 1 = 100\$, the square root of which is 10. As you add points in Artistry, your chances will quickly increase.
For my previous example of 50 and 25, you'd get \$50 \cdot 25 = 1250\$, the square root of which is 35.3.
The main downside to this method is that in a tabletop game, it's extremely hard to calculate without a calculator. Even with one, it requires a more complicated understanding of math and is more time consuming. If you put this in a rule book, there will be people who won't understand what you want them to do. For something like a video game where it's calculated by the software, that isn't a problem.
(Thanks to Peteris and Vatine for the suggestion!)
Alternative - Minimum/Maximum
A very simple method for combining skills is to use either the minimum skill in the two of them, or the maximum skill in the two of them. The maximum means you're just using the skill you're better at, while the minimum means you're using the skill you're worse at.
In the case of the minimum, it simulates the idea that you have to succeed on what you're weaker at in order to accomplish the goal. This lets you do it in a single roll, and is very easy to understand. It also has some issues, in that if you're extremely good at Stonecarving and so so at Artistry, your Stonecarving gets ignored in this system as you only roll on your lower one (Artistry).
Because of that, I don't think it really accomplishes what you intend very well, but it's ease of use is a significant upside over the other suggestions.
(Thanks to Neil Slater and Ellesedil for suggesting.)
Best Answer
Yes, a d100 is the same as 2d10 with one as the percentile.
A d100 goes 1–100, a d10 goes 0-9. Neither allows you to roll a 0, because of the way you count a percentile dice. (00 on the percentile and a 6 on the other dice forms 6, 00 on one and 0 on the other is 100, no option will result in 0.)
Do remember to use different colors of dice, else you will find things getting confusing quickly.
As an extra to the answer, it just occured to my why you asked the question. I guess you were wondering because 2d10 is not the same as a d20?
This is because (asides from one being a range of 2–20), there are multiple ways of getting the same result. If you roll 2d10, you can get a result of 7 by having: a 5 and a 2; a 6 and a 1, a 3 and a 4, etc. Because there is more than one way to get a ‘7’ result, you are more likely to get a 7 than a 2, which you can only get with 2× 1.
In a situation with a percentile dice and a ‘normal’ one, there is only one way to get every result (1 in a 100, to get 54 you need 5 on the percentile and 4 on the normal, no other result will work).