Is Kurapika the only person who has 2 types of Nen?
When Kurapika is being taught about Nen, his teacher states that 1% of conjurers and manipulators also develop Specialist abilities. This means that Conjurer/Specialists and Manipulator/Specialists exist, but are extremely rare.

It is also worth noting that Emperor Time is a specialist ability that, while active, gives Kurapika 100% proficiency in all 6 types of Nen, at the cost of using 1 hour of his lifespan for each second it is active. 

It is entirely reasonable that someone else may have a similar ability, such as a specialist who can change their Nen type each morning, giving them access to a Hatsu associated with that Nen type, or a Specialist who gave themself access to another Nen type with their Hatsu. Obviously, these would require appropriate limitations, but such is the core nature of the Nen system.