[Saru]

As far as I know duplicate meta description has a warning note in the Webmasters Tools only for the the same domain and it refers to at least two pages from the same domain that have duplicate meta description. This happened to my website once.

For different domains I think the problem is different. Still I haven't found a clear statement about this but: consider that you don't have any meta description on any of the two sites. Then on searching results the search engine would pull a sample from your page text to embed in the description place. Consider also that on the two sites you have duplicate text. That's a warning for bot. Is logical for me that the same could happened if the meta were in the description place and they were the same.