It is possible to use both extensions, but unfortunately, only the first one you use for an image will get the RAW. The second extension will get a JPEG generated by the first extension. So if you use DxO first for lens correction, you will not get access to the RAW Processing adjustment in RAW Power. If you use it the other way, then RAW Power will get the RAW and DxO will get a JPEG made by RAW Power.
I believe using RAW Power first is the best choice because DxO can perform lens correction on the JPEG created by RAW Power. Please note that RAW Power has built-in lens correction for some cameras, so depending on which camera you have, you may not need DxO. If you want to see if your camera gets lens correction, you can download the trial from our home page.
While extensions are generally non-destructive, this does not apply to sequences of extensions. Once you edit with a second extension, you cannot go back to the first extension and change your edits.