In this paper, the performance evaluation of the tag to tag (T2T) communications using UHF (Ultra High Frequency) passive RFID (Radio Frequency Identification) tags is tackled. The T2T communication system is performed directly between a reader tag (RT) and a listener tag (LT) in the presence of an external electromagnetic (EM) field. Consequently, this kind of communication system implies the use of low RF signal levels, which makes it very sensitive to the noise, to the arrangement between the tags (i.e. spacing, tilt and shift) but also to the position of the source. The performance evaluation of this communication system is based on the study of the modulation depth, an indicator of the separation between the two modulation states and, indirectly, of the T2T communication quality. Considering scenarios in which the two tags are in the same plane but with the RT tilted and staggered with respect to the LT, our study demonstrates that the modulation depth can be kept higher for different separations between the tags by choosing the proper angle of incidence of the RF source with respect to the tag's plane. The proposed approach is studied through electromagnetic simulations and the obtained results are confronted with measurements. Index Terms-backscattering, passive tags, UHF RFID, tag to tag communications.