Absolutely, and it's definitely recommended.
Just to update you on terminology, there is no such thing as a BDC in
Windows 2000 and above.
The biggest thing to consider is DNS. I would recommend switching the DNS
on the existing box to Active Directory-Integrated.
Having installed and patched the OS, install DNS on the server, but (for
now) point the new server's DNS (in the TCP/IP config) at the existing
server. Run dcpromo to add the new server to the domain as a domain
controller.
When this is done, check that both servers have an identical copy of the DNS
zones and then switch the new server's DNS to point to itself only. In the
DNS config, make sure the server has a forwarder.
You will also want to make the new server a global catalogue. There no real
need to shift the Operations Master roles around on a two domain network, so
I'd leave those as they are.
Once you're happy with the setup, update your DHCP (assuming you're using
that) to include the second DC's DNS settings in addition to the first so
that things will still work if the first DC is unavailable.
Hope this helps
Oli
