Hi stavros sotirchos,
Unfortunately the current version of RadChart for Silverlight does not provide support for such customization (there is no style exposed that would allow the customization of the specific axis item template). We will forward your feedback to our developers so they can consider improving the behavior and the customization options for the next official release of the control. Also, we are planning to add multiple YAxis support for Q3 2009 later this year (around November) so you will be able to achieve your desired appearance out-of-the-box.
the Telerik team