In two servers at the same time

I don't know exactly how I did it, but I have an idea. I pressed my own server and clicked Join, immediately hit Cancel, and joined Mineplex, and I was in both servers at once. I've tried recreating this multiple times after recording the video, but could not do it.

 

Video:

https://www.youtube.com/watch?v=8O6wd5F3l2g