Moderator note: Moved to the VCDX forum area.
Yes it is true that if someone is a VCDX already then they dont have to defend the second one (there is a limitation for people who passed the VCDX-NV during the Fast Track route)
As for another track they have to pass both VCIX if they dont have a last version (VCAP5 ) VCAP in that stream. If they have both they can choose either, if they only have one (VCAP5-DCA for eg.) then they need to do the VCIX design exam and then can submit a VCDX-DCV (to keep to the examples) as passing the VCIX would give them the underlying VCP.
To answer your example, I have VCDX-DCV but would have to pass the VCIX-NV before submitting for VCDX-NV but wouldn't have to defend it