The easier way to go about this is to build the new platform as part of the original station. If you hold down Ctrl when building the new platforms, you should get the option to add the newly built station to an existing one. The advantage of this approach is that there will still be, in effect, one station. If you build your tracks to split trains between the platforms correctly this will definitely enable you to load faster. The stations article on OpenTTD Wiki has more information: http://wiki.openttd.org/Stations
If your station is too big - the limit is 12x12 squares by default, you'll have to build a new station. The problem with this is that the new station will receive goods separate from the original station. The amount of goods given to each station depends on their station rating, which will be evaluated separately for each station. In addition, you'll have to manually reassign trains to go to the new station, proportional to the number of platforms they have. Compared to a single station solution, this is significantly more complex, and is definitely not recommended unless you have no other choice.
There's no reason to believe that the two stations will get a larger proportion of the industry's output. As mentioned before, the amount you get depends on your ratings, and having more trains moving goods from the station will decrease the amount of goods waiting at a station, which is an important factor in determining station ratings.