Open the command palette with cmd + shift + p (OSX) or ctrl + shift + p (Linux/Windows) and type "split". You'll see options for Up, Down, Left, and Right split.
Using keyboard shortcuts
Split the current tab in a direction with the following shortcuts
Up cmd / ctrl + k then ↑
Down cmd / ctrl + k then ↓
Left cmd / ctrl + k then ←
Right cmd / ctrl + k then →
You can close an active split pane with cmd / ctrl + k then cmd / ctrl + w.
Move between panes
If you want to move between open panes (with the keyboard) you have to modify your keymap file. Go to Atom -> Open Your Keymap and include this: