Whether you can sell the same course on Udemy and Skillshare does depend. When you sell a course on Udemy, you give them a license to resell your content. There are two types of licenses on Udemy, an exclusive and non-exclusive license.
The license that you have will depend on whether you can sell your course on another site or not. You will need a non-exclusive license that gives you the legal right to sell your content on another platform, such as Skillshare. Skillshare doesn’t seem to have the same rules, but be sure to check before sharing your work.
The license you have on Udemy depends on if you are part of their Udemy for Business program or not. If you are, you will have an exclusive license, meaning your work can only be shared on Udemy. You can opt-out of Udemy for Business and get a non-exclusive license, allowing you to share your course on another platform.