How do you link together 2 objects, so you can edit them both it edit mode at the same time?
The only way you can do this is by joining them into one object. Select one and shift-select the other (one has to have primary focus for this to work). Then hit Ctrl-J to join them.
If you later want to separate them back into two objects, in edit mode select the verts of one of the objects (the L and B keys come in handy here) and press P to separate.
Hope this helps!