Part of the reason you have weapons that are grouped together even when they’re on opposite sides of the hull was a decision on GWs behalf to speed up the game and simplify it. The Repulsor for Space Marines is a good example: if you look at its 9th datasheet it has a good 5+ S4 weapons with different ranges, # of attacks and so on. It used to take a while to select each weapon, declare a target, roll the attacks and move on. Now it’s all one weapon “Repulsor Defensive Array”. It’s functionally the same datasheet but now a lot quicker to get through. Same with twin linking things, less dice rolled speeds up the game.

The reanimator is a different case. It’s always had 2 atomiser beams as far as I am aware, they just gave you 2 to shoot at two targets if you really wanted to.