I'm not sure if you can do it or not (though I can't see a reason why it shouldn't work) but considering the optimization factor (each and every JavaScript slows down your site) I suggest you to add this other user to your Analytics account instead. If you don't want them to touch anything then you can give them read-only rights - rights can also be given site by site if you don't want them to see the stats of your other domains.
from what i know no. you need to have one script per page only but there may be a way to trigger it out. google it
Yes you can have two scripts on one page and you can also grant access to the other user from your analytics account where you have to mention is google account id. This way you dont need to paste the code twice and you can share it with n no of users. To do this simply log in to your Analytics account and you will find a link called User Manager next to Add Website Profile from where you can add new user. Also you will be able to decide which website statistics you would like to share. Regards, Atul