There is no evidence that they communicated with each other (however Jeremy Varone has an excellent book comparing the RAF to the Weather Underground called "Bringing the War Home").

That said, there are surprising connections between the two organizations that somewhat helped give rise to these groups: the SDS in Germany and the SDS in the US. Despite their identical acronyms, these two Student organizations were not affiliated; But Martin Klimke's "the other alliance" very clearly demonstrates that a prominent German SDS member travelled to America in the mid 60s and was helpful in drafting the Port Huron Statement (along with, apparently Jeffrey Lebowski)

Here's an interview I did with Klimke a few years ago: http://www.baader-meinhof.com/podcast-6-martin-klimke-interview/

But were the members of the of the Red Army Faction working with or communicating with the Weather Underground? No. Not to my knowledge.