For the merger between Philadelphia-based Saul Ewing and Chicago-based Arnstein & Lehr to work, the math had to check out.But adding one firm with four offices and roughly 135 lawyers to another with 11 offices and about 250 lawyers was more than just an addition problem.“When two firms are merging, and all you end up with is one plus one equals two, the whole process is not really worth it. You need more than one plus one equals two,” said George P. Apostolides, vice chair of the new firm’s …