wddm allows sharing resources over applications through these share handles though the link i provided is not the best example of usage of them (i’ve just got into this stuff lately) but aero’s taskbar thumbnail previews works that way (and ms provided a higher level api (in the dwm) for accessing/replacing them or creating new ones).
this says also wddm has the capability to share resources so not just textures or framebuffers but vertexbuffers or complete meshes as well (i guess that works through multiple hardwares too but i’m not sure about it). it’s in the windows sdk so it should be vendor independent.
and here’s the reference: http://msdn.microsoft.com/en-us/library/dd327290.aspx
this whole thing is terribly documented though. it’s obscure and it seems ms doesn’t really have a robust idea about its practical usage (like syphon targeted rather vj’s and av performers and other realtime purposes ms hasn’t targeted anybody (and it doesn’t look like they will ever target the same audience as syphon)