We need to look at the audience of ELL in comparison to that of ELU. The nature of ELL means that the answers here should be inherently different to those on ELU due to the differences in our target audience. This is much the same as stacks like Ask Ubuntu, ServerFault, and SuperUser, where questions there may be on-topic for each but will attract different answers on each.
ELU is geared towards the expert and advanced level questions about English and the answers will generally be written in a scholarly fashion as they are coming from linguists and experts. These answers may be difficult for a non-native speaker of English who is learning the language to easily understand and get the information they need.
Where a question is posted on both sites and is on-topic for both, I think we should keep it open. There are a number of discussions on MSO (2 3) and P.SE about this and it seems that there are some mixed opinions.
As far as I'm aware there is no way to close a question as a cross-site duplicate, and we shouldn't be doing that anyway. We can add a comment to a question that links to the similar question on ELU (for reference purposes) but then we should let the discussion continue.
It should be fairly obvious when a question posted here really does belong on ELU, especially where the question is of a scholarly nature or is asking about the evolution of the language, etc. In these cases - those questions should be flagged for migration (once we're out of beta and we can) and dealt with on the target site where it will be closed as a duplicate and merged where necessary.
For now, as long as there isn't a blatant copy & paste from ELU to ELL, I think we can let those sorts of questions through and let our community answer them.