If you want to passing some value, the common approach are using querystring or IMC. Querystring is the easiest way and simple to do. But you can't pass a complex data type, just string or object that you should serialize. IMC offer more flexibility but need extra code to implement. Since IMC can be 'read' by other module in the same page, then you should provide unique name for every process and catch that value from other module that implement IMC too.
You can alternatively using Javascript to do like that. But make sure the name of your control is unique. Because at runtime everything in client can be accessed using javascript.
I give you an example, if you have ModuleA and ModuleB. You put ModuleA in LeftPane and ModuleB in ContentPane.
In ModuleA you have a javascript function like below :
function Show() {
document.getElementById("spanInModuleB").innerHTML = "Some value";
}
And in ModuleB you have a span like below :
<span id="spanInModuleB" />
Then ModuleA and ModuleB dan communicate using client side. It's just an alternative. It is often that you should combine with javascript. But once again, this approach is not about passing data as you see in code behind. But if you create an object in javascript, you can directly accessing DOM element in your module and then using Ajax to postback to server. I hope you got the idea.
HTH.