Do you have control over the apps? If so, just have them 'open a channel' to each other. Perhaps you can just use a socket, to write to one from the other, or a named pipe, or something similar. It'd what I'd do, if I was writing both of them.
The easiest way is to use ServiceHost to expose a WCF service. You can not directed access an object from a process from another process. The only option is to send a message to the other process interpret the message and execute a action.
Here you can find a quick example: http://blogs.microsoft.co.il/blogs/davids/archive/2009/05/31/a-hands-on-lab-for-wcf-moc-6461.aspx
Hope this helps.