If you are not familiar with child themes concept read the following first:
My previous article Sharing users across multiple WordPress installations explains how to prepare for our current task. In order to be able to access multiple WordPress blogs with only one login we have to have common users across those installations. This article will deal with additional requirements necessary to accomplish our goal.
After reading Justin Tadlock's article Installing two WordPress blogs with the same users I have decided to implement this method in two of my test installations. Well... it did not work. Why?