How can I share Cache folder in two different place(one at main domain and other at it's subdomain)

Well as I have posted earlier too...I have created a site in two languages.One with URL www.mainDomain.com(English) and other with www.fr.subDomain.com(French).Both are done in CakePHP,in french I have just changed the views of it to French.The problem I am facing is of the Cache.I have app/tmp/cache folder in two different places.Once a cache is written, it does'nt updates on the other's folder.That's the reason why some of the values does'nt get updated when I am calling them from Cachethrough AJAX.While loading the page it shows the updated value,but when page is fully loaded,it is showing the previous value.How can I rectify this?How can I make both Cache folders as common?Is there any way to do that?
please Help...!!!