Scott, do you only have two MA's? If so, when you reply, instead of sending to Nursing, just cc the second MA and you can avoid the issue you describe. That also avoids the issue of them having to remember to check the Nursing box as well.
Naeem, it sounds like what you are talking about is not a daily thing, but viewing others' boxes when they are off. Perhaps logging into their account and checking their messages when they are sick is the logical way to handle that now.
I am not saying that the ability to view multiple inboxes is unnecessary;just that there may be ways around it.