Whatever you call them both Dreamweaver and Expression Web use DWTs and they behave more or less the same way. I say more or less because Expression Web DWTs will work out of the box with no changes in Dreamweaver, updates of pages attached to the DWT will all happen whenever you make a change and save it (assuming that you say yes when asked if you want to update attached pages.)
The reason this happens without any intervention on your part is because Expression Web uses the exact same syntax as early versions of Dreamweaver. So for backwards compatibility all version of Dreamweaver up to the present version (CS 5) understand and honor that syntax which was last used back in Dreamweaver 4.
Starting with Dreamweaver MX Macromedia changed from
<!-- #BeginTemplate "site.dwt" --> & <!-- #BeginEditable "doctitle" -->
to
<!-- InstanceBegin template="/Templates/site.dwt" --> &
<!-- InstanceBeginEditable name="doctitle" -->
The change wasn't just because Microsoft added DWT support in FrontPage but also because new capabilities were added such as the ability to nest DWTs and have editable attributes aa well as conditional regions.
So if you are using a Dreamweaver DWT that uses any of those capabilities you will not be able to use that DWT in Expression Web.
If you are using only basic DWT functionality you can change the syntax in your Dreamweaver DWT and use the DWT in both Dreamweaver and Expression Web. I have done so on many occasions since Dreamweaver does not support ASP.NET and some things are easier to do in Expression Web while others, especially with some of the Project Seven extensions I have it is much better to use Dreamweaver. The good news is that you can use both programs on the same site but realize that you should keep good back-ups just in case if you chose to do so.
