A and B are simply the two ends of one tunnel. The choice is arbitrary and neither end has a special role — GRE has no client, server, initiator, or responder. Use A on the first server you set up and B on the second. The only differences are that A takes the first address in the tunnel subnet and B takes the second, and that the local and remote endpoints are mirrored between them. Every other setting — key, MTU, TTL — must match exactly on both servers, or the tunnel will appear up while carrying no traffic.