Tandems I go about 20 feet for the aft lure (four arm lengths about depending on your size, arm span is equal to height). And 14 feet for the forward rig (lighter one). Then I attach those two lengths and rigs toa 3 way swivel (with swivels on the rigs if they arent weighted because 3-way swivels dont spin good at all). Then I put 2 feet to an N-Line and then another 2-feet to a swivel to be attached to the mainline.
For those lure lengths, if you hate long leaders, you could take an arm length off of each length
All this is done in 50 lb. cajun, but you could get away with 40 (I wouldnt go any lower then that if you anticipate skates, because tandems get expensive!). If you dont anticipate skates, you could get away with 30. My main lines are all 30, usually mono, which usually provides enough shock protection to get away with smaller leaders, but the sizes I gave above work best to keep my tandems from getting tangled.
Always put the bigger rig on the longer line, smaller on the front.
For rigs, I use Tsunami bunker swim shads (great look and action), in trophy season, a 9 inch on the back, and a 7 inch on the front, and since they're so light usually I use a 4 ounce inline and drop it back 14 or so guide lengths for around 30-40 feet, and vary depths off of that for other rods.
I also throw in an occasional ruby-lipped bucktail with a shad attached in place of the 7-inch shad, if you ever see super striper bucktails that are shaped like a cashew with a spinner, they work just as good and are half the price.
DONT forget about big parachutes (not in tandem), White or Chartruese of course, dont be afraid of how big some of them are either, I caught a 38 incher on a 10 inch total length parachute, and he swallowed the ENTIRE lure.
Also, try to avoid snap swivels, they can bend out and you'll lose the big fish. Make sure are your knots are sound, I use improved clinch knots. I also bring a hand gaff for skates after an incident last year (in addition to a necessary fishing net, because you never know if they'll spit the hook at you when your buddy mess's up and releases pressure on the line). When we catch skates, we usually catch quite a few, we caught 4 in 2 hours one day, and one left me a nasty scar in my hand.
I caught quite a few fish during the summer also using nothing but tandems, and using mono for my main line which is considered a big no-no in trolling due to its stretchiness.
The key is to know where all your rigs are (what depth). If you see the rod tip look like a fish is nibbling on it, reel in some, your dragging bottom. I vary all my lines (usually 3 on a 17 ft boat) about 3 or more guide lengths each to prevent tangling.
Always make long sweeping turns never sharp or quick, otherwise, your rigs will hit bottom and or tangle. If you have 3 or so rods, rig the weight the same, and set the guide lengths (guide lengths I refer to is each time the line guide makes a pass to either side) to varying lengths coming up from the farthest rod back, which will be the first one you set. This way you cover all depths the fish are in, and can adjust if necessary later. Depth finders are very nice in helping you after you get a feel for how deep you set your rigs with different guide lengths and weights/setups.
With fish on, always reel in the rods with rigs in front of your battle, I leave the ones set behind it out.
When you bring the fish to the boat, most people get annoyed getting him in due to the long leaders, you'll have to pull the line up to the side of the boat, and use a fish catch net! About 3 feet wide, it makes everything so much easier!
There's a lot more to it also, but the most important thing I learned I think is troll where there are some other boats! If you know where the fish are and the boats arent, thats even better, but chances are someone got out there and found them before you. Trolling in other spots is also great as I found some fish that no one knew about last season and caught tons.
Good luck this season, Im probably missing some key points but I hope this helps!