Two-way backlinks means reciprocal links. If the backlink is from the high pr page with related category then it is very useful....
Google sees the one way links as the best. That is you are linking to a page on www and its not linking back to you. If you are mutually linking to each other then its not one way linking and carries much less value as compared to one way links. This should look very natural to any analytic because one page (S1) would link to a site ( S2) only because it contains some thing of importance that is relevant to some content on his page. But if there is a link form S2 pointing back to S1 then it looks more like a collaboration among the owners of the two sites to increase PR. That is why Google gives more weightage to one way links.
yes still you can get seo advantages from 2-way link...but make sure it is come from relevant website......
The others have pretty much answered your question. It is worth something, just make sure that you are linking to related sites. Google loves relevancy and high quality content so go for those.
It is very import because it signifies a nice relationship with the website you exchange your links with...
Two sites communicate with each other then transfer the data from one to other is called two way back links Example:- A share to data B B share to data A