Assuming it were permitted by licensing, does the following configuration for getting both SBS premium servers running on the same box work technically?
- Install the allowed 'physical' instance of SBS 2008 'add on' server on the bare metal, with only the Hyper-V profile (and possibly the GUI-free 'core' installation mode). Do not have it a member of a domain.
- Install the instance of SBS 2008 'main' server atop #1
- Install the allowed 'virtual' instance of SBS 2008 'add on' server atop #1
Aside from contention on I/O, memory, CPU, etc, are there any reasons not to do this?
Well both the SBS and add-on pack include a W2K8 Standard licence and each standard licence entitles you to 1 host and 1 VM via their 'Virtual Image Use Rights'. So yes, you'll be covered by this completely.
Yes. Quoting the license agreement:
So your "Standard" SBS can be installed on a Hyper-V Server (note that I said Hyper-V Server, which is a free product from Microsoft; it is not part of or on the SBS DVDs. You have to download this separately from MS).
The "Premium" server can be another VM on the same Hyper-V Server, or at your option, on a physically different server, including running on that server's bare metal or as a VM under a hypervisor (such as the free Hyper-V server mentioned above).
You may not install a Core version of the SBS server, add the Hyper-V role, then create two VMs within it for the "Standard" and "Premium" SBS servers.
From a technical standpoint it's a good idea to separate the roles into at least the two VMs. Most modern hardware can easily handle running two VMs, though it typically needs a bit more RAM and HD space. CPUs are rarely a bottleneck these days.