Let's say
 

your trolling two rods off a twenty footer in anywhere from 10 to 40 feet of water and you hookup up on one or both rods. You've been trolling at about 1 1/2 knots or thereabouts. Should the person at the helm slow down? Stop? etc. Obviously, I do not have any trolling experience to speak of and I get out with someone relativly new to it as well. Thoughts, tips appreciated.

Depends on the situation, how many guys on board, are you in dangerous waters where you could drift into trouble if you power down?
If I was by myself, I would turn boat out of danger then power down and fight fish while keeping a close eye on things. With other anglers on board, I'd maintain the helm to let them fight fish and slow boat or even stop if needed, Depends on situation.

Make sure your rods are locked in. Maintain speed to keep the fish hooked. Once the angler has the rod you can back down. and like f-18 said be aware of your surroundings. What is the safe thing to do? If you are in the open ocean don't worry unless it is crowded, but in some areas someone should always be in control of the boat incase you need to move quickly.

depends on the size of the fish ;)