Hello VolcanicMink,

You do it in yhe right way.

When you have done your copy, effectively, both shoes ares combined in a single objet.

(save your project first)

For example, if the original one was the right shoe, just select all the polygons of your "new" left shoe, select Edit/copy (you copy all the polys), select Edit/delete (remove all the polys of you left shoe) and finally Edit/Paste (you create a new object).

You will see a new group in the scene editor: click on it and rename.

Now, you should have two separate shoes.

Save the two object.

Hope it helps