Good afternoon, probably on the forum this issue has been raised before, but I did not find a suitable topic.
There is a 50x100 board, I would like to place 2 identical boards on one and make the size 100x100. How to make a fault line correctly to break one board into two?
What element should it be and in what layer should it be?
Brand new interactions and interfaces. 
Easy to use and quick to get started. 
Please see the Tutorial and read about "Panelize".
If you choose to panelise by hand, put the V-Cut lines on the Board Outline layer and not the Mechanical layer:
https://easyeda.com/forum/topic/V-Cut-lines-must-be-drawn-on-the-Board-Outline-Layer-cbe1471cf93d410ea3141e431d52bb8b