I agree with skeetbum, doubles have their place, but when the effort exceeds the reward time to scale back to singles.
As for weight, it may take some experimentation to see what works best. Might be worth your time now to rig up a method to quickly add/remove weight while on the water. I like 3-ways and snap swivels. Tie up some weight with a snap swivel, attach to the three-way. If too light, or too heavy, swap out to another pre-rigged weight.
As for trolling up and down stream. Personally, I would pick one direction and troll in that direction only. It will take more weight to push upstream than it will to drift down stream. If you go back and forth (up/down) you will constantly be switching weight. As for Crappie, they are lazy and will sit out of the current and wait for food to drift on by, dart out, grab the food, and dart beck out of current to eat. They won't be looking downstream to eat.
HOI Crappie Club
Where family and friends come to compete for a little more than bragging rights.
Quick, someone teach me how to fish so I can win this tournament!!!