If i understand you right you want two halfs with a space inbetween so you can join then with a straight horizontal extrude or elongated cube.

1 change to face select and delete half the model.

2 hit insert on the keyboard and move the pivot point of move tool away from the remaining mesh half the distance you want your gap to be and hit insert again to complete the pivot move.

3 mirror in the x or -x depending on which half you saved.

4 select both halves and combine.

5 center pivot and then either extrude one side to the other or add the cube as required.