I actually liked it with two driftwood pieces. Your tank is pretty long and your largest driftwood piece is too small to just stand alone. That smaller piece acts as a counter balance to the other side-- especially with how you had it positioned. If open space is desired move each piece to their further side. I would do this as it would follow the aesthetically pleasing rule of thirds. The only other thing I would do is try to find smaller rocks to place near the base of your larger ones to create a gradient effect.