将组合因子列分割为r数据中的两个因子列的最有效方法是什么?

I have a large data.table (9 M lines) with two columns: fcombined and value fcombined is a factor, but its actually the result of interacting two factors. The question now is what is the most efficient way to split up the one factor column in two again? I have already come up with a solution that works ok, but maybe there is more straight forward way that i have missed. The working example is:I have a large data.table (9 M lines) with two