From the server's perspective, the graph of conversation history from group A and group B merge. From the client's perspective... depends on the client I think,

From the server's perspective, the graph of conversation history from group A and group B merge. From the client's perspective... depends on the client I think, but most seem to display the messages from the other side of the split all at once when they're first received. Clients don't currently make it clear when messages came from the other side of a long netsplit, but the data is there on the server so in principle they could. I think the client API might need some changes before that'd be possible though.