Yes, this is possible. There are a couple approaches that might work depending upon your sites and DNS.
One approach to direct users from each site to their respective Expressway pair is to use a different service discovery domain for each site. Let's use east and west sites as an example. Users from the east site would be configured to use east.example.com for service discovery, and that would resolve to an A-record that points to the East Expressway-E. And users in the west site would use west.example.com for service discovery. And this domain is not something the users need to be aware of, it can be provided in the jabber-config.xml (under VoiceServicesDomain policy) that clients download from CUCM TFTP servers, or is bootstrapped into the client via MSI (jabber for windows) or URL configuration (for non windows jabber clients).
The other approach would be to use a GEO-DNS service that will direct users to the closest Expressway based on their location. This would allow all users to use a single DNS domain for service discovery, but it will require a GEO DNS service to load balance DNS responses based on the user location. For this to work, the distance between your sites (east and west) needs to be far enough that the GEO DNS service can actually differentiate between users at each site and direct them appropriately. There are plenty of Geo DNS providers out there to evaluate, I recently worked with CloudFloor (https://www.cloudfloordns.com) which allows SRV load balancing and is a clean solution, whereas some providers will have you load balance the A-record returned in the SRV response, which is possible too. There's some good info in the Preferred Architecture CVD on this as well, https://www.cisco.com/c/en/us/td/docs/solutions/CVD/Collaboration/enterprise/12x/120/collbcvd/edge.html#56595
Endpoints or users connecting over the East Expressway C/E pair should be able to communicate with users connecting over West Expressway pair as long as RTP media can stream between Exp-C hosts in the east and west sites. And Jabber clients will be able to chat as well.