[CentOS] C6.5 - combine two DVD isos into one tree?

I have the two DVD isos for CentOS 6.5 mounted as loopback, on the mount points /centos6 and /centos6a, and am trying to use them for PXE/KS installation.

The problem I have is that the packages in /centos6a/Packages are not found by the installer.

The host is a CentOS 5 system. Is there a way for me to overlay the second DVD on the first without having to copy all the packages from both isos into a new directory?

Or is there a standard path I can use to mount the second DVD that the installer will use for packages that are not found on the first?

Or any other ideas? I'm sure I can't be the first to stumble over this!

Cheers Tony