Hi,
I can't provide an authoritative answer because I haven't tested this scenario or seen a conclusive answer to the question when it has been asked previously. However, I can tell you that the NPS role does not benefit from clustering. To load balance NPS,
you need to use remote RADIUS server groups and proxy the authentication requests. See
http://technet.microsoft.com/ru-ru/library/dd197433(WS.10).aspx.
If you have multiple NAP health policy servers, you will need to have a way of replicating policies among the various servers. A better idea might be to have a single NPS server, or a group configured as described above using a remote RADIUS server group.
Each of the DHCP servers can then be a proxy and forward authentication requests to the main NPS server (the NAP health policy server) for approval.
-Greg