I would avoid trying to use threads for this. Most games you create, especially simpler ones like Space Invaders, don't need multiple threads for things to visually appear to be happening at the same time. In fact you should almost never need to use multiple threads at all. If anything, having multiple threads will greatly complicate your code and they are prone to causing issues that are really hard to debug.
At a high level, most games work by having an infinite loop, where each time through you clear the screen, draw everything, and then display it to the player. (See
the tutorials if you aren't familiar with the basics). Remember that the player won't see objects move at the very moment the code tells the object to move. The player only sees things move once the window has been displayed. So for 2 objects to move "at the same time", all you need to do is move both of them between your calls to the display function.