Yes, it is safe to use the Gemini as a replacement for the 2 lax balls taped together.
It is much more effective in some areas.
The 2 YTU balls in the mesh bags are balls that are softer than a lax ball.
For some areas a lacrosse ball may be too hard.
When the videos are shot there may not always be a gemini available to use.
There are episodes where the gemini is used.
Different tools work differently for different people.
In certain areas one tool can be more effective than others.
Some will come down to what the person experiences the best results with.