That only entries from the first \addbibresource are proposed after pressingOk, that's what happens also to me when RefTeX cannot find the database file.
Indeed. The embarrassing thing is that now it works: RefTeX finds both databases and I didn’t do anything. It did work on another machine with almost identical configuration when I tried late last night (once it occurred to me to try) and worked without problems on both from then on.
I suspect that this, far from being RefTeX related, is due to a cloud syncing service (SpiderOak, if it matters) I use. Both machines are on it, and the location $HOME/Library/texmf as well as my working dir are among the ones being synced. For goodness knows what reason, one of the databases, the second one loaded, was not being found. Now, though, I can’t reproduce the problem.
Sorry for the noise,
Department of Mathematics, Florida State University
Tallahassee, FL 32306-4510, USA * * aldrovandi at math dot fsu dot edu