In XAML databinding in WPF, it is possible to do simultaneous data binding conversion for multiple values using the IMultiValueConverter interface. In the Silverlight core libraries there is however only support for single value conversion using IValueConverter.
In one example Silverlight application I developed, I wanted to compute the resulting amount in a specified physical unit, given the amount in another physical unit. A Silverlight multi-value converter would have come in handy, and gladly enough the job had already been done.
Colin Eberhardt has provided an extensive solution together with detailed usage instructions in his blog. I have applied Colin's implementation and it works flawlessly.