I have a joomla instance and a cakephp instance in my site. What I want is to have a single login form for both the joomla and the cake instance.
Is this possible? Possibly there is a plugin that allows this?
You could use a Joomla-CakePHP bridge to execute Cake app in your Joomla instance.
If you want the two to work separately I think you can set up an authentication in Cake to user Joomla database and the data stored there when you are authenticating users.
You can achieve your task using session cookies. Be aware that Joomla! writes by default session's data in database not in file This link can help you to figure out how to proceed.