How can I share a cache folder in two different places (one at main domain and other at its subdomain)

Well as I have posted earlier too...I have created a site in two languages, one with URL (English) and the other with (French). Both are done in CakePHP. In the French version, I have just changed the views of it to French.

The problem I am facing is with the cache. I have app/tmp/cache folder in two different places. Once a cache is written, it doesn't update the other's folder. That's the reason why some of the values don't get updated when I am calling them from Cachethrough AJAX. While loading the page it shows the updated value, but when the page is fully loaded, it is showing the previous value.

How can I rectify this? How can I make both cache folders behave as one common cache? Is there any way to do that?