Well, this is a question with many possible answers. In my experience, I would always recommend a second photographer (not just an assistant, more on this latter) if the wedding party is larger than 50-70 people and of course for Indian weddings (where you may need some times two-second photographers). Another question would be why do I need a second photographer? the short answer isn't just to get more pictures but to get pictures from another angle and with a mix of styles between the lead photographer and the second shooter. Take for example the bride entrance, if the main photographer is on the front capturing the walking it would be very hard to obtain a (clean) picture of the groom at the same time, here's where a second photographer even without messing the other photographer's shoot (maybe using a long lens) can get the other angle. The other key feature of a second shooter is that he or she can be a backup for the main photographer, he or she can get the missing shots or more candid shots for example when the lead photographer is taking portraits.
Most of the times you can get an amazing photograph from the second photographer because he or she are walking the scene composing images without having the pressure that the main photographer has. Being a second shooter doesn't mean that the experience is less compared to the lead photographer. Another point that I mentioned before, is the assistant the same as a second photographer? No, many photographers advertise an assistant is included on the collection they offer but an assistant like the name implies assist with non-related photography tasks like carrying bags and light equipment or gathering people for a group shot. So, do I need a second shooter or not? I would say if you want to capture every possible image and angle of your destination wedding, or if you plan on to create a nice album, or if your wedding party is very large a second photographer is a must. As I always advise couples to not get a decision based solely on money but on what you really what or need. Additional photographers can raise the price of a collection on 25% or more but I would say it's paid off with the return you get. Tip: when you book a second photographer ask to see his or her portfolio, do not assume the images would be the same as the lead photographer (they will not) it doesn't mean low quality but a different photography style.