Open up the TeamSpeak 3 app, choose 'Bookmarks' and add two different servers you want to connect to. Connect to the server of your choice.
Once you are connected to the first server hit the back button in the upper left of your screen, then tap the 'Bookmarks' button.
Now choose the second server and hit 'Connect to server'. You are now connected to 2 different TeamSpeak 3 servers at once.
In order to switch between these two servers just tap the servername (back button) in the upper left corner of your screen, then tap 'Home'.
You will be presented with a list that shows the servers you are connected to, just choose one of those servers by tapping on its name and it will become active.