I've never seen or tried this but if you're going to "y" it heres a product that might make it easy. Looks like you'd be putting it in upside down , so the end will need to be crimped. Hopefully your problem is the hole cant handle the water, and not that the downspout cant handle the water. Prolly worth a shot before before replacing the whole thing. http://www.gutterworks.com/diverters.html

Before going to the trouble of adding another downspout, make sure the gutter has not pulled away slightly from the fascia where the overflow occurred - this frequently happens when gutters are attached with spikes. If the gutter begins to pull away from the fascia, it creates a low spot where the gutter is pulling away from the fascia.

Thanks for the lead on the "Y" device to join two downspouts into one. The overflow is from the front of the gutter and I have reflashed the unit to the fascia so that is not the problem. I was hoping the gutter could be repitched to solve the problem but they tell me the pitch is adequate. I wish there was a way to determine if a 6" gutter or a second downspout opening (that merges into one drain outlet) would give me greater capacity to protect against overflow. The 6" gutter would probably look better but I am more concerned about making sure the gutter does not overflow.