Nem egészen ifthen-szerű:
If the text of your marginpar depends on which margin it is put in (say it includes an arrow pointing at the text or refers to a direction as in "as seen to the left..."), you can use \marginpar[left text]{right text} to specify the variants.
Ld. itt.
Egyébként a dokumentumosztály opció között van twoside?
Szerintem az mbox nem kell, ld. itt:
The error comes when you try to build something movable inside a box. Movable things, in this context, are floating environments (figure and table, for example), and \marginpars. LaTeX simply doesn’t have the mechanisms for floating out of boxes. In fact, floats and \marginpars themselves are built out of boxes, so that they can’t be nested.