The bait selection for a stacked line is generally a spoon because it has one hook and a small foot print. The stacked line is set at a distance that is equal to the lead length of the bottom presentation plus the bottom bait’s foot print plus the foot print of the stacked line’s bait to minimize tangles. So if the lead on the bottom bait is 15’ and you believe the trolling foot print is 5’ plus the foot print of the stacked line’s bait is 3’, set the stacker a minimum of 25’ above the bottom line. Maintaining 25’ between stacked lines will help during deployment and when a fish hook up occurs on the bottom line in that this spacing should provide enough separation for the trolling foot print of each bait as well as allowing the released line to clear the stacked presentation as the fish rises past the stacked bait. Things to consider when attaching the stacked line are lead length between the stacker release and bait, number of hooks on bait and the trolling foot print of the stacked bait. Keeping the stacked bait’s lead shorter, using a spoon with a small trolled foot print and a siwash hook will help to minimize tangles.
When lowering a stacked presentation make sure the boat is at your desired trolling speed and lower the downrigger weight slowly to the desired depth. Plastic flashers have some buoyancy and will rise if you freefall the downrigger weight, under water currents will also affect a flasher as it drops too. Maintaining a reasonable separation between the stacked lines and deploying the stacked presentation at a reasonable drop rate will help to minimize line tangles.
Personally an additional copper rig, or leadcore on an inline planer board will produce better catching results and is a lot less hassle, especially if fishing is slow. The clarity of the water these days has made stacking of downriggers an option rarely used.
Stacking is fine if you have enough guys and lines legal to do it plus the fish are fairly deep.
I wouldnt ditch 2 mid range boards these days to stack more downriggers if your only running 6/ 8 lines though. Unless the water is super warm up top and its dead down to like 70 feet.