Unless you're making some kind of super large AAA game, you should be fine with 1 thread.
There is just no way that your game needs more than 1 thread. Besides, rendering happens on the GPU, not CPU. Unless you're going through all the pixels on the screen.
The only time you should be having a second thread, is if your game has a lot of stuff to load. Than you could add another thread to load stuff and nothing more. That would make user experience better, because they wouldn't have to wait for stuff to load.
I'm using Java2D to make the game. I know how to draw 2D with LWJGL basically. I have to understand it better for me to be comfortable to make a game.
I'm using BufferedImages with createCompatibleImage so I guess I should be fine... Maybe if I get stuck at some point with the performance i'd switch to OpenGL.