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This paper addresses bounds on the transmission capacities of two coexisting wireless networks (a primary and a secondary network), where each with multiple antennas shares the same spectrum and operates in the same geographic region. In the two coexisting network, the secondary (SR) network limits its interference to the primary (PR) network by carefully controlling its active transmitter density. Each transmitter uses a subset of its antennas to transmit multiple data streams, while each receiver uses partial zero forcing (PZF) to cancel interference using some of its spatial receive degrees of freedom (SRDOF). Considering general power-law wireless channels with path-loss exponent α >; 2 and small-scale Rayleigh fading, based on stochastic geometry tools and Markov's inequality, we first derive bound on the transmission capacity for the PR networks. Then the transmission capacity of the SR networks is derived when the transmission density of PR network maintains unchanged. Using the obtained bounds, we derive their optimal number of data streams to transmit and the optimal SRDOF to use for interference cancelation.