I once was told that asking a mathematical question simultaneously at MathOverflow and Mathematics SE was bad practice. It was recommended to first ask it at one of both sites. If the question was more advanced: first at MO, if more basic: first at MSE. Only after a couple of days without an answer, ask it at the other site.
I'm facing the same situation with respect to Biology SE and Cognitive Sciences SE. In this case, the distinction is not between more or less advanced, but between two different approaches to the same question/problem concerning (cognitive) neurosciences.
Is it OK to ask the same question simultaneously at BSE and CGSE, e.g. this one: Criticism of the significance of fMRI studies?