This paper determines the hydrogen generated during the course of a severe accident in one reactor unit with crossflow through the hardened containment vent piping to the adjacent reactor unit. The hardened pipe vent systems of both units are joined at the mixing chamber at the base of the stack. Per RELAP5 code simulation results, hydrogen will retain significant concentrations in the mixing chamber regions and at the entrances of the connected pipes during the high pressure venting stage of the proposed venting scenario. The concentration of hydrogen will drop after the transition to the low pressure venting. The time required to reduce hydrogen concentration to less than 4% from the connected pipes vary depending on the sizes and location of the pipes. The results and conclusions can be used to support the HCVS design changes to provide severe accident venting capability and compliance with Phases 1 and 2 of the NRC Order EA-13-109.