The author remixed this model.
I modified the design to make it easier and faster to print (changed the thickness of some parts, connected some disconnected segments in the design, removed the small writing at the bottom, which could create problems)
Now you can get the 2 objects perfectly aligned just by running the “center model” command in your slicer.
Just remember to uncheck the “Drop down” / “Drop on build plate” command in your slicer, than set to 0.4mm the z value of the part related to the second extruder.
UPDATED (V2) VERSION:
Added some fillets to avoid the single point fill on square corners typical of Arachne engine
Connected some more sections
All credits go to LMaker, creator of the original design:
https://www.printables.com/model/129617-offset-xy-dual-extruder-idex-calibration
Refer to his design to see all the documentation.