wantmygcnow,
It says on the RTD application form (I-131) that if you are applying for a Travel Document and your current TD is valid, you must surrender it to USCIS. They want you to include it with the application package. Otherwise, you need to wait until the TD expires and then apply for a new one.
"C. A refugee travel document may not be issued to you if:
1. You have already been issued such a document
and it is still valid, unless the prior document has
been returned to the CIS, or you can demonstrate
that it was lost; or
2. A notice was published in the Federal Register that
precludes the issuance of such a document for
travel to the area where you intend to go."
Unfortunately, the rule was made under the assumption that RTDs would be processed in an orderly and timely fashion. Basically the USCIS does not want you to have two valid TDs in your possession. This would make perfect sense, IF we were not placed under extreme waiting conditions for our TDs.