Hamsters can live fine by themselves, as long as their owners interact with them and they get out of their cage daily. You don't have to introduce a new cage mate, because it's unlikely to work out.
Introducing hamsters if you're not a professional is nearly impossible, especially if they aren't from the same litter. They could very well fight. You can go for it, but ONLY if you have a second cage where you can put the hamsters if a fight does break out. You'll also want to add a couple more hideouts to the cage. Another thing, make sure your cage is big enough. A fight WILL break out if the cage is too small. The minimum is 360 square inches. If you have a cage like a habitrail or crittertrail with lots of different cages connected together I don't recommend it at all. They will fight over the different cages. If you have a cage with levels, I don't recommend it either. You need a cage that is just one big, flat, space. You'll need 2 wheels, a big enough food dish for each, etc.
If you decide to go for it, you can't just buy one and plop it into the other hamsters cage. You'll need to have a temporary second cage for the new hamster until the two get used to each other. Introduce them in neutral territory (bath tub, etc) and if all goes well put them in the first hamster's usual play place outside of the cage and if things are still going well (after a few weeks) you can try in a freshly cleaned cage. NOTHING can have the first hamster's scent on it. Also, you'll always need to have a second cage ready in case they fight. It only takes one fight, one second, to fatally harm each other or worse. Have a vet prepared.
It's very unlikely to work however, it isn't recommended that you introduce hamsters because after they are separated from their siblings, they tend to not accept other hamsters.