Hello!
From what you wrote it's not quite clear the intention you put behind this and the final result as well. So here's some questions I need you to answer to:
1) What of the post from CPT A becomes what for the posts of CPT B? The taxonomies, as you probably know, in wp are not bound to any object type but are a colelctively accessible pool of hierarchy branches, and every single type of wp object can be assigned to any of these. Also the taxonomies, here I use this in the sense that wp is using it, are not actually doing anything but to act as a basket in which to put the terms (data in general) which are what it is tied with the wp objects. So CPTA post X will become taxonomy key ("basket" to put further the terms for CPTB) or term in some other pre-existing taxonomy key. Maybe a real life set of data as illustration would help understand the intent.
2) I suppose there would be need to define the CPTA>>CPTB relationship somewhere (options page maybe) (appoint particular CPT as data container for some other CPT, as well as setting up the relationship parameters - like which field of CPTA, from the example, to use as data value etc), so not gonna be simple function only. I would say that this is better to be wrapped as a plugin itself, there's no need to use other`s plugins you can write it ourself, both from maintenance as well as portability point of view.
Kind Regards,
Dobri