I think that derivations are at a level that is too low to be useful for interoperability. The format for derivations has not changed in Guix (yet?), but you wouldn't work on derivations directly most of the time. Most of the fun happens at a much higher level where an abstraction for packages and build systems exists.

The needs of the two projects have diverged over time and the daemon in Guix is slowly being rewritten in Scheme (this has been the subject of a GSoC project and most parts already exist on the Scheme side for other purposes). I would not count on continued compatibility, nor do I think it is particularly important simply because the contents of the derivations is different, so that there would be little difference between running both Nix and Guix on the same system and feeding Nix derivations to the Guix daemon (if that even works).