Isolate the power. Drop the faces away and photograph or draw the connections of each unit.
Tile up to the edge of the backing boxes, making sure you don't overlap the edges. i.e., make straight cuts where necessary.
Once clear of the boxes, you can then refit the faces, aligning them horizontally, and try to push them close to each other to take up any gap between the faces.
Don't tile up to the face edges because it looks an ar$e. The faces need to cover any exposed tiles edges.
If the faceplate screws are too short after tiling, you can buy longer ones cheaply enough, though they may need to be cut down slightly.