A large amount of work has been invested in devising algorithms to implement distributed shared memory (DSM) systems under different consistency models. However, to our knowledge, the possibility of interconnecting DSM systems with simple protocols and the consistency of the resulting system has never been studied. With this paper, we start a series of works on the properties of the interconnection of DSM systems, which tries to fill this void. In this paper, we look at the interconnection of propagation-based causal DSM systems. We present extremely simple algorithms to interconnect two such systems (possibly implemented with different algorithms), that only require the existence of a bidirectional reliable FIFO channel connecting one process from each system. We show that the resulting DSM system is also causal. This result can be generalized to interconnect any number of DSM propagation-based causal systems.