You could do it through redirects to from x.com to y.com/login, which sets cookies on y.com's domain, and then redirects back to x.com. Either only do it on log

You could do it through redirects to from x.com to y.com/login, which sets cookies on y.com's domain, and then redirects back to x.com. Either only do it on login or set another cooking on x.com once you've done it once. (I actually work on a web property that does similar, although for different reasons.)