Just been reading about Flockport and not sure if that can be integrated with CF CNI... what could be an option is that at each site, every node has a bgp router agent like FRR/Quagga. There should be 2 other nodes which act as route reflectors to advertise the subnets to each of the same site nodes. Wireguard can them be used to build a VPN from each of the border nodes. The BGP peering between the border nodes then advertises the container routes from the other side and this way container to container communication is a possibilty.