1) You can have single EXP for two clusters provided you use the same domain for login for both clusters both internally and externally.   I have attached here a guide which i found on support form explaining the detailed configuration of such scenario. It will answer all your questions. Yes, Users in both cluster will be able to chat. As Per the Below guide Below are the restrictions https://www.cisco.com/c/dam/en/us/td/docs/voice_ip_comm/expressway/config_guide/X12-5/Mobile-Remote-Access-via-Expressway-Deployment-Guide-X12-5.pdf Multidomain and multicustomer support is limited as follows: — Before X8.5, each Expressway deployment supported only one IM&P domain. (Even though IM and Presence Service 10.0 and later supports Multiple Presence Domains.) — As of X8.5, you can create multiple deployments on the Expressway-C, but this feature is still limited to one domain per deployment. — As of X8.5.1, a deployment can have Multiple Presence Domains. However, this feature is in preview status only, and we currently recommend that you do not exceed 50 domains   *** Please rate helpful post; Mark "Accept as a Solution" if applicable Thanks, Haris   ... View more

Just collect the Even Viewer Logs from CUCM. and Look for Unregister and Register Events with Reason COde. YOu can search for Messages with keyword EndPointUnregistered. You could see KeepAliveTimeout Reason 13 https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/err_msgs/8_x/ccmalarms861.html UC_CALLMANAGER-3-EndPointUnregistered : An endpoint has unregistered Device name [String] Device MAC address [String] Device IP address [String] Protocol [String] Device type [Enum]Device description [String] Reason Code [Enum]IPV6Address [String] IPAddressAttributes [Enum]IPV6AddressAttributes [Enum]LastSignalReceived [String] CallState [String] *** Please rate helpful post; Mark "Accept as a Solution" if applicable Thanks, Haris ... View more