RJ has run them without. It will make the board loser and easier to spin, and faster down the line. You will lose tons of drive that way, if you do try it as a twin, you may want to try fins like the Scimitars or 3/2/1s to give you a bit more drive than standard twin fins. That board is really designed to be a quad with tons of drive, I refer to RJs SS Pro as the "pumper" because its speed is generated by pumping the board. If you watch a video of say Chris Kinsey Jr or Chris Wolter building up speed to get air, and then watch maybe James Walker or RJ riding his soulcraft, you will see the difference.
So even though the board will be faster down the line and loser, if you really like that setup/style, you should look into a Soulcraft Jetty, Inland Flyboy, TWP Signature, TWP Bomb Pop, those boards are designed to really take advantage of that type of setup more. The SS Pro really shines when you ride it as a quad. One thing I will add, that board is VERY sensitive to fin setup. There are fins we have put on the quad SS Pro that make it feel very sluggish, and others, that don't look radically different, that give it more down the line speed and drive.
Chaos, CK Jr, and Chris Wolter can give you some good tips on what quad fins to try for a setup you may really like.