That is not a issur M’Dirabannan. There is a Cherem on those who do it (I know that is lawyering, but it is a major distinction).
What, exactly, is this “major distinction?” I fail to see one. Saying that something is a cherem is the same as saying that it is forbidden.*
The Wolf
* Lest you say that the distinction is that the second marriage is, post facto, valid, remember that the marriage of a kohen and a divorcee is also, post facto, valid, but yet forbidden.