Wouldn't such a case involve using two separate cobbler servers. It is
not uncommon to separate lab and production networks. In such
an environment the production DHCP and cobbler servers in production
would not be accessible to dev/qa users anyway. There could be
separate cobbler servers for each environment although they could
share the same install trees and repositories.
I don't know how common the use case is, but I'm in favor of not
worrying about it and letting people just manage this with clever use of
kickstart templating in most cases.
The DHCP server could also be listening on two interfaces, that's common
enough. The --dhcp-tag functionality was done to accomodate this (being
able to template two sets of subnet definitions in dhcp.conf)
A dual-homed cobbler server is not uncommon, anyway.