Personally, I would be transparent with the applicants.  The only risk I see is that employees of both companies might become upset about the limitation.  If it hadn't been communicated earlier, it could become public by sharing it with the applicants.  Granted, I am not very experienced so I hope some others respond here!

I would be very cautious of these types of formal agreements between companies. There have been lawsuits over them. Here is a blog post about them from a legal perspective.  http://blogs.findlaw.com/california_case_law/2014/10/more-silicon-valley-anti-poaching-lawsuits-filed-3-things-to-know.html

Is this anti-solicitation, or anti-hiring from each other's organizations?  I had a similar agreement with another HR Manager. We actually shared clients, but their employees worked with insurance (backend)  and our employees worked with the patients directly (frontend).  So they knew the same software platforms and spoke the same language.  That said, we had an anti-solicitation agreement but we didn't have a "no hire" agreement.  We gave each other a heads up but wouldn't prohibit hiring if the person applied and was the top candidate for the open position.  We actually hired several of each other's amicably this way.