So many things to wonder about how a HHB film or series would be able to do Cor and Corin as twins.

Then there's hair color. It has been common for Narnia films or series to depict the characters with hair color that is different than in the books.  In the books, Susan is described as being dark haired and Lucy is described as being blond. Yet in the BBC, their hair color somehow got switched- Susan is blond and Lucy is dark haired.

For Cor and Corin, there's not a particular hair color I have in mind for them. It would be idea to portray them as blondes, as described in the book. Though they could end up with different hair colors, maybe a ginger. I think portraying them right will still count for something, regardless of what hair color they decide to go with. 

Many fan castings on myCast I have seen have casted Shasta/Cor and Corin as the same actor. I can see why this can work because they are twins, though can see this having complications with terms of filming should HHB be live action, especially when they interact with one another.