In the context of the UHF (Ultra High Frequency) passive RFID (Radio FrequencyIdentification) technology, recent works have proposed a new paradigm demonstrating the possibility toperform short distance tag to tag (T2T) communications. This paper presents a study that focuses on theperformance evaluation of T2T systems. Without loss of generality as to the methodology followed, dipoleantennas are considered here (assumed ideal in the theoretical part, then printed for the rest of the study).The results obtained show that the modulation depth, which can be considered as an evaluation metric forthe T2T systems, is strongly impacted by three main parameters: the switching impedances (here chosen asshort-circuit and open-circuit); the geometrical configuration constituted by the two tag antennas formingthe T2T system; and also, the position of the external source on which the backscatter communicationrelies. As the modulation depth is very sensitive to these three parameters, which are more or less directlyrelated, it is very difficult to predict the communication quality for a given configuration. For example,when the two tags are in parallel and the source is symmetrically positioned, the modulation depth shows adecreasing trend by oscillating when the distance between the tags increases from 0.1 λ to 1 λ (λ being thewavelength): with values going from 70% to less than 5%. As a consequence, from a very complete set ofscenarios considered, the presented study helps to illustrate and explain the interactions of these parametersand their impact on the modulation depth. And at the same time, the paper provides guidelines for definingthe specifications of such a system based on quantified results. The set is completed by introducing the BER(Bit Error Rate) as an evaluation metric by considering then a more complete T2T system (i.e. listener tagincluding a non-coherent envelope detector).