How to select a publishing venue for interdisciplinary research?
The answer of jakebeal seems pretty good to me, but there is another option (to be considered with care, as the line between wise publishing and salami publishing is rather thin): to publish two papers in both A and B. The first paper would describe in depth the technique of the method and could assume knowledge in A; the second paper would assume the conclusion of the first paper (proofs and evidence and technical discussions being there) and would explain how good the method is for the B-problem you are considering, i.e. it would focus on the application of the method. The point of publishing two papers is that community B might have a hard time getting into the technical details of the A-heavy method, while you need to have them documented somewhere.

The first paper might only get published in a lesser venue if community A is not appealed to much by it, but I guess that if community B is excited by your work you might be able to publish the second paper in a first-tier journal or conference. And, as mentioned in a comment, it might be that you have a good starting point for a career in B -- one need not stay in the same field for 40 years, and switching can potentially happen anytime, given the opportunities.