Looking around for a web server that is quick to configure, I came across the Sun Cobalt Raq 4i. I already know that it can be used as the primary or secondary DNS server. However, I'd like to know whether you can also use it as the primary and secondary DNS server at the same time?

I suspect that the answer would be yes, but I want to make sure before purchasing one. And yes, I am aware that it would be ideal to actually get 2 servers, preferably on different network segments.

Yes, technically it most definitely can be used for both (but as you pointed out, this isn't the ideal method).

Of course if you're using the RaQ to run your websites as well as your DNS servers, and it goes down, what good would a seconday DNS server on a seperate network do you? :)