If you start off with a form like below you have several options to straighten up the margins:.
You can you alt drag the form, change to white and if needed bring some elements to the front/back.
Or you can add a stroke to the bottom shape. In the 'stroke panel' select "more options" in the top right and then choose "align stroke to outside"
Or click bottom element and select "Object / Path / Offset Path" and colour the new form white.