Depends on the size of the group also.
If you're playing 4 or more, gambling, playing tags, it's generally frowned upon to throw multiple shots.
The last hole can be an exception as you are done for the day.
playing with one buddy it is ok to throw 2 drives on multiple holes if it doesn't hold up groups behind you. you are also not permitted to actually play both drives, only 1 and generally the first (unless you are not keeping score feel free to play the better one to speed up play).
The second shot needs to be done quickly, after you drive you should be picking out a disc and be ready to shoot immediatley after the other guy.
In terms of not helping your buddy, you should tell him prior to driving that you are no longer going to help him (obviusly he would not need to help you anymore), but unless you say something, I think it's safe to assume you owe him some help looking