You can use LayerToCanvasX LayerToCanvasY CanvasToLayerX CanvasToLayerY expressions.
For example, to convert X/Y coordinates from layer "A" to layer "B":
newX = CanvasToLayerX("B", LayerToCanvasX("A", x, y), LayerToCanvasY("A", x, y))
newY = CanvasToLayerY("B", LayerToCanvasX("A", x, y), LayerToCanvasY("A", x, y))