Time-Domain Reflectometry (TDR) RFID tags are passive, chipless tags that use discontinuities along a transmission line to create reflections. The discontinuities may be designed to produce a bipodal signal encoded with the unique identifier of the tag. When multiple tags are co-located and interrogated simultaneously, multiple access interference degrades the ability of the reader to detect the tags accurately. Reader detection can be improved by using spread spectrum signatures as the unique identifiers to limit interference. This work evaluates the ability of Gold codes and Kasami-Large codes to improve detection performance of a passive, chipless TDR RFID system. Simulations were conducted for varying numbers of simultaneously interrogated tags using synthetic tag responses constructed from the measured waveform of a prototype TDR tag. Results indicate that the Gold Code signature set outperforms the Kasami-Large Code signature set and a random, naïve set for simultaneous interrogation of less than 15 tags. For larger numbers of simultaneous tags, a random set performs nearly as well as the Kasami-Large Code set and provides more useful signatures.