a,b,c;

..: layout=overlap {
    [layout=overlap]{
        a<-b [color=red];
    } {
        b->c [color=green];
    };
};
..: layout=one_by_one {
    [layout=one_by_one]{
        a<-b [color=red];
    } {
        b->c [color=green];
    };
};