Your scenario as I understand it would really be quite difficult to implement in RadRichTextBox. We do not support tracking changes to the document in the first place. In addition, there will be indeterministic cases with tracking multiple users' changes to the same pieces of the document.
As for the spans splitting and merging - spans are split when different style settings are applied to different parts of a span and are merged when adjacent spans's style properties are set to the same values. We have not met the need to have nested spans in the document and I am not quite sure how that will help with your scenario.
You can try using document positions that track changes to the document (they are created using the two-parameter constructor of the DocumentPosition class with the second parameter set to true) in order to keep the beginning and end of the text that is modified. These document positions need to be manually disposed of at the end of their use by invoking their Dispose() method, otherwise memory leaks will occur.
Let us know if we can help further.
the Telerik team
Let us know about your Windows Phone 7 application built with RadControls and we will help you promote it. Learn more>>