We recommend two different servers mainly due performance but also because it makes easier the administration. Off course if you have a big server and only few users, you could handle the two services in the same box.
Just keep in mind that out of the box BigBlueButton has a web server (nginx) configured to listen the port 80 (as any other web server has by default). You can deal with the configuration of moodle on nginx or if you want to install a different webserver like apache, you would need to put either of them to listen to a different por, hence to do extra configuration.
We as BigBlueButton developers and commercial providers never do that, not even for development, but yes, we have learn of several deployments of both services in one box. So, the use of two servers is highly recommended but it's not mandatorily required.
Regards
Jesus Federico
Good reply Jesus.
Paul, see also this FAQ entry:
http://code.google.com/p/bigbluebutton/wiki/FAQ#We_recommend_running_BigBlueButton_on_port_80
Regards,... Fred
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BigBlueButton Developer
Thanks guys, the information is very helpful!