Why do some airports have to put 2 jetbridges on one aircraft?
It is not strictly necessary. But it is done in order to save time. 


More points of passenger loading leads to faster loading, potentially reducing delays, airport charges etc.
In some cases, there is a seperate jet bridge for first/business class.
For large aircraft like A380 etc, is quite common to use multiple jet bridges as use of a single one may lead to quite large loading time.