Before answering, there is some info that needs confirmation. Does Store A and B sell the same or similar products? Why?
If both stores sell different products, then you redirect store B's traffic to store A, then your bounce rate will increase dramatically since people want to buy store B's product but got directed to A which is different.
However, if they sell the same products, then it is definitely do-able. The best pracitce is that you should use 301 redirect on each page of store B website and redirect them to store A's pages that sell the same product. You want to direct people from viewing store B item 1 to store A item 1. Here is an article about Matt Cutts explanation on 301 redirects. http://www.mktdojo.com/matt-cutts-discusses-301-redirect-limits You don't want to redirect people to store A's home page when they want to check out a specific item. You should direct them to a similar page in Store A.
As for Risk and benefits, when you use 301 redirect, it won't transfer ALL link juice and authority but probably 99%. Another risk may be it will take Google longer to follow the 301 redirects and crawling speed. Some people might get confused, they want to visit store B but got directed to store A.