Datacolor User to User Support Group.

The reason I think it's better to target the second monitor's WP to
the first one's CIExy numbers:

When you target both to the same white point (6500K or D65 for
instance) the results may be on different sides of the target, so to
speak, increasing  the difference (Delta E) between the two. It is
better to get the actual numbers of one and try to get the other one
as close as you can.

Note:
You can't use the  K number (CCT) of the first monitor as a target for
the second monitor . You have to use CIExy. A white point target has
to be a defined chromaticity.