I would say that questions on ELL should, in general, be of a sort that a person learning English might well ask, or a question such as might be of benefit to a person learning English. There is no rule that says a native speaker cannot ask a question. However, if thew question involves a technical point of usage of a sort not likely to be interesting or helpful to a person learning English, it might get better answers on ELU. It is not uncommon for a question to be migrated from ELU to ELL, or from ELL TO ELU, so if a question is asked in the "wrong" place, it is fairly easy to move it to the "right" place. So I wouldn't worry overmuch about which site to post on. But please don't post the same question to both at once.