I have two webapps WebApp1 and WebApp2 in two different domains.
One can use invisible iframes to get the cookies. Let's say there are two domains, a.com and b.com. For the index.html of domain a.com one can add (notice height=0 width=0):
That way your website will get b.com cookies assuming that http://b.com sets the cookies.
The next thing would be manipulating the site inside the iframe through JavaScript. The operations inside iframe may become a challenge if one doesn't own the second domain. But in case of having access to both domains referring the right web page at the src of iframe should give the cookies one would like to get.