The monitoring of modern large scale infrastructure systems often relies on complex event processing (CEP) rules to detect security and performance problems. For example, the continuous monitoring of compliance to regulatory requirements such as PCI-DSS and NERC CIP requires analyzing events to identify if specific conditions over the configurations of devices occur. In multi-organization systems, detecting these problems often requires integrating events generated by different organizations. As events provide information about the infrastructure' internal structure, organizations are interested in reducing the amount of information shared with external entities. This paper analyses the problem of detecting policy violations in network infrastructure systems managed by two organizations (e.g., a cloud user and a cloud provider). We focus on CEP monitoring systems and we introduce two protocols for selecting the events to share between the two organizations to ensure the detection of all possible policy violations. Our experimental evaluation shows that reciprocal information sharing between the two organizations significantly reduces the amount of information to transfer. In our SNMP monitoring test case, we obtain a 80% reduction in the information shared by any single organization.