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GtkPaned is a special type of container widget that holds exactly two widgets. A resize bar is placed between them, which allows the user to resize the two widgets by dragging the bar in one direction or the other. When the bar is moved, eighter by user interaction or programmatic calls, one of the two widgets will shrink while the other expands.
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#include <gtk/gtk.h>

static void
destroy(GtkWidget *window, gpointer data)
{
    gtk_main_quit();
}

int main(int argc, char *argv[])
{
    GtkWidget *window, *hpaned, *button1, *button2;

    gtk_init(&argc, &argv);

    window = gtk_window_new(GTK_WINDOW_TOPLEVEL);
    gtk_window_set_title(GTK_WINDOW(window), "Panes");
    g_signal_connect(G_OBJECT(window), "destroy", G_CALLBACK(destroy), NULL);

    gtk_container_set_border_width(GTK_CONTAINER(window), 10);
    gtk_widget_set_size_request(window, 320, 240);

    hpaned = gtk_hpaned_new();
    button1 = gtk_button_new_with_label("Resize");
    button2 = gtk_button_new_with_label("Me!");

    g_signal_connect_swapped(G_OBJECT(button1), "clicked", G_CALLBACK(gtk_widget_destroy), (gpointer)window);
    g_signal_connect_swapped(G_OBJECT(button2), "clicked", G_CALLBACK(gtk_widget_destroy), (gpointer)window);

    gtk_paned_add1(GTK_PANED(hpaned), button1);
    gtk_paned_add2(GTK_PANED(hpaned), button2);

    gtk_container_add(GTK_CONTAINER(window), hpaned);
    gtk_widget_show_all(window);

    gtk_main();
    return 0;
}
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