In this example we are going to pack two rectangles in a box, and have a separator in the middle.

So we start we the window, background, box and rectangle creation, all pretty normal stuff:

//Compile with:
//gcc -g separator_example_01.c -o separator_example_01 `pkg-config --cflags --libs elementary`

#include <Elementary.h>

EAPI_MAIN int
elm_main(int argc EINA_UNUSED, char **argv EINA_UNUSED)
{
   Evas_Object *win, *bx, *rect, *separator;

   elm_policy_set(ELM_POLICY_QUIT, ELM_POLICY_QUIT_LAST_WINDOW_CLOSED);

   win = elm_win_util_standard_add("separator", "Separator");
   elm_win_autodel_set(win, EINA_TRUE);

   bx = elm_box_add(win);
   elm_box_horizontal_set(bx, EINA_TRUE);
   evas_object_size_hint_weight_set(bx, EVAS_HINT_EXPAND, EVAS_HINT_EXPAND);
   elm_win_resize_object_add(win, bx);
   evas_object_show(bx);

   rect = evas_object_rectangle_add(evas_object_evas_get(win));
   evas_object_color_set(rect, 0, 255, 0, 255);
   evas_object_size_hint_min_set(rect, 90, 200);
   evas_object_size_hint_weight_set(rect, EVAS_HINT_EXPAND, EVAS_HINT_EXPAND);
   evas_object_size_hint_align_set(rect, EVAS_HINT_FILL, EVAS_HINT_FILL);
   evas_object_show(rect);
   elm_box_pack_end(bx, rect);

Once we have our first rectangle in the box we create and add our separator:

   separator = elm_separator_add(win);
   elm_separator_horizontal_set(separator, EINA_TRUE);
   evas_object_show(separator);
   elm_box_pack_end(bx, separator);

Note:: Since our box is in horizontal mode it's a good idea to set the separator to be horizontal too.

And now we add our second rectangle and run the main loop:

   rect = evas_object_rectangle_add(evas_object_evas_get(win));
   evas_object_color_set(rect, 0, 0, 255, 255);
   evas_object_size_hint_min_set(rect, 90, 200);
   evas_object_size_hint_weight_set(rect, EVAS_HINT_EXPAND, EVAS_HINT_EXPAND);
   evas_object_size_hint_align_set(rect, EVAS_HINT_FILL, EVAS_HINT_FILL);
   evas_object_show(rect);
   elm_box_pack_end(bx, rect);

   evas_object_show(win);

   elm_run();

   return 0;
}
ELM_MAIN()