This question makes some assumptions that may be (are) erroneous. Carrying two laptops from the US and into South Korea, and then back to the US several months later isn't, or shouldn't be, of concern to either Customs or Border Protection of either the US or South Korea. In these days of online connectivity, many of us travel with multiple electronic devices all the time. I carry a laptop, a tablet, a smartphone (android), many flash drives and memory chips, and I am lighter than many I know.
Your problem will be if you do get flagged for interview in S. Korea, by Border Protection, and if you tell them truthfully (and you should always always always be truthful, because being caught in a lie will cause you to be flagged from further travel, at the very least), that one of your laptops is for working an online job, then you are admitting you will be working while in the country on a student visa.
Just because it's an online company, and the work you do from them can be done from anywhere, does not mean it is legal to do everywhere. If you are earning income inside a country, you may be expected/required to pay taxes on the foreign earned income. The fact you are working, at all, may or may not be allowed while on a student visa.