Hi,
One constraint about DocumentBuilder is that it merges styles, and takes the first style in the list of source documents. If you want to keep both styles in both documents, then you will need to rename the style in one of them. You can write a small program to do this – not hard. Need to change the style name in the document.xml part, and also in the styles.xml part.
Be aware that if you have other styles based on the style you are renaming, then the reference from the derived style also needs to be updated. But most probably you are not facing this situation.
At the time I designed DocumentBuilder, I made this decision – it more or less meets peoples needs, but occasionally, we will find an issue such as this. Renaming styles before merging should solve your issue.
If you want to see what needs to be done to rename styles, use the approach shown in screen-cast #13 in the following series:
Introduction to Open XML
Cheers, Eric