There are different ways to communicate between usercontrols. Here is a sample of that using events. Basically an event is raised when something happens in one Usercontrol an event is raised and Page which contains the UserControl subscribes to the event and passes the relevant details it to a different UserControl
you can download the sample here
Lee's corner 
Hi,
Maybe you want to see another way to do it – I’ve blogged about it here on my blog here:
http://blogs.microsoft.co.il/blogs/alex_golesh/archive/2008/07/10/quick-silverlight-tip-communicating-between-two-silverlight-objects-on-a-single-page.aspx
Regards,
Alex
Alex,
I like your approach. Thanks for sharing
Always welcome 🙂
Alex
I agreed with you
I found the below mentioned post, which uses event aggregator to communicate.. can you please check it once and let me know which one would be better to use. I am really new to Silverlight and Asp.Net coding.
http://dotplusnet.blogspot.com/2010/08/how-to-communicate-between-two-user.html
regards,
Kailash
If you are using Prism, then it is good. if you just want a simple solution without extra dlls, then you dont need to go through that route.
here is the solution
http://reddyinfosoft.blogspot.in/2012/05/interaction-between-two-user-controls.html#more