Client1, Client2, Server, Backend1, Backend2;

[layout=one_by_one_merge] {
    defstyle arrow [color=green-50];
    Client1->Server;  Server->Backend1;
    Server<-Backend1; Client1<-Server;
} {
    defstyle arrow [color=blue];
    Client2->Server;  Server->Backend2;
    Server<-Backend2; Client2<-Server;
} {    
    symbol block left at Client1+ right at Client2- <<<
        background.color=lgray+40;
        col { box Client1, Client2;}
        box [line.type=dotted, fill.color=lgray] {
            box Server;
            col { box Backend1, Backend2;}
        }
        arrows use color=green-50;
        Client1->Server->Backend1;
        arrows use color=blue;
        Client2->Server->Backend2;
    >>>;
};
