Yes, it's pretty much that simple but you will also need to change your stabs since there are now two keys to stabilize. That means you might have to take pretty much the whole board apart depending on what kind of stabs you have. It will likely be 2 × 2u stabs

You would need to change from your current stab to 2x 2u stabs.

Although Idk why you would bother if you're going to map both to space, most people map one to backspace or sometimes return.

See other comments.

I'd like to add that 3U spacebars are available with 2U and 3U stabilizers. Make sure you have the correct wire size.