While you are free to state the terms by which the GFDL should be interpreted for GNU documentation, this is not always the case. We have in the past seen cases where copyright holders have interpreted seemingly unambiguous statements in a pathological fashion (see Pine, for instance) - in the GFDL case, the wording does not make it clear that it is the intention that the license may be bound as a separate volume. If this is how you wish the license to be interpreted, clarification of the license would be helpful.
I think it is clear that a printed work can consist of multiple volumes, but clarification might not hurt. I will think about it.