Marlow Moss. Composition with Double Line and Blue, 1931.
Piet Mondrian. Composition with Double Line and Blue, 1935.
Yes they knew each other and collaborated. They developed the double line cooperatively. You'll see that fact mentioned in every piece discussing the British Moss. I have never once seen it said anywhere in writing about Mondrian.
(Marjorie became Marlow and lived as a man from 1919 but is still referred to in all literature past and present with the pronoun "she.")

