Splitting a segment into two

You could try using Random Bucket numbers! One segment could be < 5000 and the other > 5000. Screenshots below! May not be exactly 50/50 depending on the size of your audience, but certainly faster.

Not sure what you're then doing with the segments, but there is also the A/B testing functionality in campaigns that would randomly split them into variants, or in Canvas there are Experiment Steps or Audience Paths that could both accomplish this.