With this Plugin you can print one object on an other one. It simply changes the "Z" values of the second object in the Gcode.
You can get the Plugin under the following link. File:Printonobject.py
!Attention! this plugin is work in progress, use at your own risk.
Simply print a first object as a base. If the print is finish don't remove the object from the Table.
Load the second object in Cura and enable the plugin Printonobject. Then config Cura as you like to print the second object. Enter the height of the first object and deactivate the skirt (for Cura - 13.06.4 under Expert --> Open expert settings... --> Skirt, set Line count to "0"). Make sure that the second object is in right position and size to be printed on the first one. The printhead will move to home position and therefore needs free space in the front left corner.
Save the Gcode file and open it with an editor to check if the first line was inserted by the Printonobject plugin.
If the first line was not inserted, the plugin failed to edit the GCode. In this case better don't use the file because the printhead could crash into the first object.