The easiest way to do this is simply to slice two files and concatenate the gcode.
I wish slicers had an even smarter mode: Reduce free-space moves. When printing a castle with 4 turrets, the slicer could be smart enough to know that after printing the main part of the castle, it can then print the turrets one at a time because they won't hit the print head.
If the turrets were a bit taller, and would collide with the print head, then you could still print half of all the turrets, followed by the other half of all the turrets.
End result: Less print time, less retractions, less moves over free space (stringing).