When an employee leaves an employer for a period of time (but not exceeding two years) and returns must the employer once again seek to obtain information it may have received previously from other employers?

No. If the information received previously is still on file with the employer, the employer need not seek to obtain the testing data again.
However, the employer must seek information from all other employers for whom the employee performed safety-sensitive duties since the employee last worked for the employer.

DOT Part 40 Question and Answer dated 1/2002