Don't know for sure but I would think that for a 2-machine deployment you'll need a standard web server certificate and a separate FMS certificate...
Wim is correct - you'll need a second certificate for the webserver on the WPE worker machine.
The one installed via fmsadmin will secure all traffic to the main database server and the admin console website on the primary machine.
I've tested that I only need one certificate for Pro/Go traffic and WebDirect web traffic in a two machine config.
The part that isn't documented is that you've got to install the same certificate on both machines. Only need to request once. Use the same response file and serverKey.pem on both machines. Just have to manually install (twice) using the CERTIFICATE command line option. You do have to reboot after installation. Confirmed that the certificate was installed under IIS (can't test Apache, but should work the same)
Thanks. I had read this in another forum as well and really appreciate the confirmation. We will be giving this a shot in the next week or so and will report back.