DDD - confused on how to implement a use case involving 2 aggregate roots

I have two aggregate roots: User and Sketches.

In my application, users can clap for a Sketch. In the service layer, it looks as so:

class SketchService {
  constructor ({ SketchRepository }) {
    this.sketchRepository = SketchRepository
  }
  
  async addClaps ({ sketchID, author, count }) {
    const sketch = await this.sketchRepository.findById(sketchID)
    
    sketch.addClaps(author, count)
    
    await this.sketchRepository.update(sketch)
  }

  // other use cases
  // ...

This works great. But what if every User has a karma and every time a user claps for a sketch, the author should receive +1 karma?

Now the issue is that SketchService would have to now contain UserRepository; I am not really sure if that's a good thing.

Then it would look like this:

class SketchService {
  constructor ({ SketchRepository, UserRepository }) {
    this.sketchRepository = SketchRepository
    this.userRepository = UserRepository
  }
  
  async addClaps ({ sketchID, author, count }) {
    const sketch = await this.sketchRepository.findById(sketchID)
    const sketchAuthor = await this.userRepository.findByUsername(sketch.author)

    sketch.addClaps(author, count)
    sketchAuthor.addKarma(count)
    
    await this.sketchRepository.update(sketch)
    await this.userRepository.update(sketchAuthor)
  }

I am new to DDD so I am not sure if this would be the best approach. Should I use domain events? Is using two repositories like in this situation a good approach?

Thank you.