To reiterate what's been said,
- no
rudder; directed trust,
- smaller props; very little prop walk, if any,
- pivot point farther aft; takes more thrust to spin,
- props closer together; takes more thrust to spin.
You can certainly pivot by splitting the clutches on an outdrive or
outboard. You might have to use a LOT of throttle though. Getting true sideways motion (no
wind or
current helping) would be quite a feat. I won't say it can't be done, or there will be 100 testimonials claiming to have done it. But testimonials are the poorest form of evidence.
Basically, sideways movement using just twin screws on an inboard leverages the fact that the
rudder behind the forward-moving prop has more influence, and some prop walk. Without these (and without resorting to letting the
wind or
current do its thing) you really can't "walk" any boat.