This is very, very good idea I would maybe just change the time line since it's not possible for Big J to know anything about the children tears extracting process because she just followed SUM into the abyss & down into the dungeons.

We could go with it in this way:
1.Cut scene is first showing the BJ entering the dungeons through the bottom of the abyss & then switching to LJ, who is given a potion by the mouse.
2.In the first part of the dungeon level J's have to find each other, with BJ starting from the bottom of the dungeon & LJ starting from the dungeon tower.
3.Once they unite we go with cut scene where LJ is telling the BJ about the strange machine that was extracting his tears & about the big monster (BUM) that was standing next to SUM, talking about his plan to extract tears from every child in the valley.
4.After the cut scene the second part of the dungeon level is starting & J's have to find & destroy the tears extracting machine.