If you have to do it, make sure that the field is clear of last years straw. If you spread your flax straw last year, those bunches will hang up and plug your knife and push and cutting time. The second risk is late flushes of flax that can still be green at cutting time. Even if desicated, the lentils will dry down before the flax straw. We have intentionally intercropped the 2 and that works but is tricky, but with volunteers, you don't have control of maturity the same, you just don't know when it will germinate, how thick it might be, or manage the fertility so it matures properly.