The most common reason why people run 2 characters is for coil.

Some players just want to do coil twice, with the same or different people.

Those who do it with the same people are usually going for the double loot method. 4 main accounts paired with 4 secondary accounts, two different teams. Loots per team goes to the 4 main accounts. In this way, gearing is speed up much faster.

Another scenario is for coil selling. One of the raids in our FC have a full alternate account just to sell coil carries, so that gearing for their main account jobs will not be affected.