Your log in ID is in no way connected to either of the two sites you have mentioned.
You are free to create log in ID of your choice provided the particular site accept your ID, you can create identical log in ID's at both the sites, it won't create any problem and further to say what you contribute in both the sites will earn you separately as long as the contribution is not duplicated or copied from other sources.
You can contribute about same subject with different texts in your own words in both the sites, which may bring you cash credits and points separately depending on the quality of the post, but never copy the identical texts in both the sites, it is not allowed.
Hope this might have cleared your doubts and looking forward for your valuable contributions.
N K Ravishankara