Currently, the games do not have native support for SLI or Crossfire graphics. The game does not report to these what card should render which 3D elements on screen and because of this it is left to the drivers but this can actually sometimes lead to reduced performance because of the extra calculations etc.
Typically, we recommend to players using an SLI/Crossfire setup that they use "Alternate Frame Rendering" (AFR) which can act as a form of "safe mode" for SLI/Crossfire systems when running a game that does not support them. (Note for Crossfire users: ATI might use a different descriptor for AFR mode, the default mode when support is not enabled in the game being run is "Scissor" which is not the same as AFR.)
There is good information in this post here (it is on the LOTRO forums but is easily adaptable for DDO) that details SLI/Crossfire and related configurations including AFR and it can be a very useful guide to getting the game to perform on these types of systems.