Dual speedometers doubles the chances that one will actually be working when it is really needed. For a competition ski run the boat will typically pull the slalom rider up and then fairly quickly line up to the course. If the speedo isn't working right the rider won't get a valid pull and his run doesn't count. If he just used all his energy on a run that didn't count he will be pissed.
There is no "proper" speed per se. For competition slalom skiing there are a range of speeds mixed in with shorter rope lenghts to keep making the course harder and harder. You may have heard the term "38 off" which means the rope has been shortened the maximum amount, or in other words, the best of the "pro" rider setting.
For recreational skiing the "best" speed is whatever the rider is comfortable with. For a beginner rider that is often just fast enough to get them on top of the water. For a child on double skiis that can be pretty slow (~15 MPH). As they get more comfortable increase the speed, or better yet let them decide (use the thumb up or thumb down to signal speed changes)
For wakeboarding the trick for a rank beginner is to pull them just fast enough to get their board above water until they get stable. For a small kid this might be just above idle! For an adult it might be 16 MPH. Plowing through the water will be tiring but it will allow them to have the board in just about any orientation and be okay. Once they have the board turned towards the boat speed up a bit.
Once a wakeboarder is past the basics and is working on wake jumps it becomes critical that the pull is consistant, both speed and direction. Slight variations in speed, while approaching the wake, can really throw the rider off. If you don't have a Perfect Pass to control the speed for you then you will need to keep a very critical eye on the speedometer and continuously adjust the throttle as the boat simply will not maintain a constant speed for wakeboarding.
Having the boat turn will change the shape of the wake and will either accelerate the rider or slow him down depending on if he is on the inside or outside of the turn. Watch where you are going, and watch the rider, make your turns when the rider is in a neutral position. If you are following a canyon, river or slough it is much better to make sharp turns then hold a stright line rather than making large sweeping turns.
Oh, one last thing: don't make power turns. If your rider falls, and there is not another boat threatening to run him down, keep your direction constant until you have dropped to idle before turning around. This will avoid sending rollers off ahead of you, which your rider will have to ride through once he is up again. It is also more courteous to other boats/riders.