Colin Eberhardt describes an update he applied to his MultiBinding solution for Silverlight 4 - he introduced support for ElementName and TwoWay binding.
A few months ago I posted an update to my MultiBinding solution for Silverlight 4. This technique allows you to perform the same kind of multibindings which are possible in WPF, where a property value is bound to multiple sources via a special value converter that implements the IMultiValueConverter interface, which describes how these values are combined. This update proved popular once again, and I received a few requests to add support for ElementName and TwoWay binding. I like challenge! This blog post describes how these two features were implemented, but if you just want to grab the code, you will find the link at the end of this article.