I am seriously being considered for two positions, with the hiring decisions to come in the next couple of weeks. Should I inform the organizations that I am seriously being interviewed by another group, or do I wait until I receive maybe two offers?
I think the hiring manager or HR can assume, since you are a valuable candidate and have a lot to offer, that you're being pursued by many firms. In terms of disclosure etiquette, I don't think you have any obligation to say anything at this time.
For me, the only times I would feel obligated to disclose anything about my job search is (1) after I have accepted an offer with a different company. Then I think it would be nice to let other companies know that I am no longer available. Or (2) if I believed that the information I shared would improve my chances to receive an offer.
Certainly, when I communicate with any company I always make sure they know that they are my top choice (even if they're not). If you're uncomfortable saying a company is your top choice (when they're not) at least be able to name several key reasons why you want to work for that company. : )