For email (before the days of the cloud), you would get a email filtering product that could include multiple engines, BitDefender, Kaspersky, etc. however it's one main product installed to the system, utilising the multiple engines it has licensed from the vendors.
It's also possible... to deliver email/web through multiple servers. Each one with a different AV / Filtering product that checks/cleans before delivery to the final mailbox server.
File servers, you'd have AV on the server and on the client, you should be concerned about entry points, email, web, USB stick / CD ROM and ensure each has it's own protection, in combination should have as well protected a network as possible.
Running multiple products on a single server/PC, not a good idea, unless most are passive/manual and only one "active"/installed/running in the background otherwise it will slow down/crash the system running two or more full AV "installed/active" products.
Biting the hand that feeds IT
