2 beads are common on "target guns" regardless whether they are Trap or Skeet guns. A center bead and then a front bead - and like others said the idea is to stack them one on top of the other in a figure 8 - but they are only used to make sure you have the gun mounted properly - so its not canted or mounted differently than normal. After that - ignore any beads on the rib.
You can shoot both games with 1 gun - if you want. Many Trap guns are set up to shoot a pattern that is 60% high / 40% low based on the point of impact - so you can float the target above barrels / some Trap shooters even like a 70/30% split. But there is no reason why you can't shoot Trap with a gun that patterns 50% / 50% either - meaning you can't float the target over the barrels - you have to cover the target and kill it. Most Skeet, Sporting Clays and bird hunting guns are set closer to 50% / 50% - so they shoot a little flatter.
The idea on shooting Trap and Skeet - focus only on the target - feel the lead, don't measure it, pull the trigger, and follow thru to execute the shot. As you execute a shot with a shotgun ( different than deer hunting with a slug ) the idea is to put the shot column in front of the bird - then the bird flys thru the pattern - and is killed. To use a fixed "sight" would defeat that purpose. Hitting moving targets - clay or feathered - the shot starts as you see the bird, mount the gun, feel the lead, pull the trigger and follow-thru. If you don't follow thru - you are not executing the shot - you are stopping part way thru. The shot does not end when you pull the trigger - you follow thru and your eyes still stay on the target / then you shift your eyes (without moving the barrel ) and without taking your head off the comb if you can - find the next target - then move the gun and execute the 2nd shot. Same issue on Skeet pairs, Continental Trap where you can fire 2 shells, Trap Doubles, Sporting Clays - or shooting Quail, etc - see the bird, mount the gun (using beads to make sure gun is level ) / find target and focus on leading edge (not the butt ) and not the "whole target" focus on the beak ..., feel lead, pull trigger and follow thru before you fully execute the shot. To put a Red Dot on a target shooting moving targets would really be defeating all these principles - and may work on some angles / but it won't work very well on abrupt crossing angles, passing shots with birds moving from behind you - overhead - and down range / or birds heading at you and overhead / or birds flying in an eliptical pattern from right to left or left to right - where you have to create a rectangular target box in front of bird and your barrel may be pointing to lower left corner of box as bird is moving left and falling . A fixed sight, like a red dot, will probably make you "punch" at the bird and stop as you fire ( kind of like a rifle shot ) which isn't productive.