A cutter tool for cutting the innermost cell of two concentric cells as found in cell-in-cell coverings for architectural openings includes an elongated main body with lateral side extensions defining a space into which an inner cell of a dual cell can pass. Within that space, cutting blades are positioned along opposite sides of the main body of the tool so that relative movement of the tool along a dual cell cuts the inner cell along opposite sides thereof without damaging an outer concentric cell so that an anchor bar can be positioned within the dual cell in engagement with the outer cell for connecting the outer cell to an upper rail or bottom rail of the covering.