In Minecraft 1.12.2, when on the Server screen, clicking quickly twice on 1 server and then on server 2 about 5 or so times makes the game lag a bit (Not "not responding" though) and then connects you to both servers. The way that it does it is that for example:
-When you look around you see the Mineplex Lobby
-You collide with the blocks on where they are on Hypixel
-And you get the chat from both servers.

Typing commands seemed to do nothing and writing normally in chat seems to just be 1 of the 2 servers.

I can go into more detail if necessary.