Basically the load average is dependent on the process queue length. That is the number of processes waiting on the processor to run - this is different than the list of processes itself. On a xMP machine the next process switch that occurs the queue will go down by the number of processes (but if the running aren't done they go back).
So a dual processing box, you shouldn't see any problem with a load up to 2. In the real world, it can go higher without problem - usually. It is dependent on the types of processes you're running. On a web server I would think you could get to 5,6 maybe even 8 at the extreme. Past that you're gonna see slow downs.
I've seen loads of 12-14 on a single RISC box and it was still usable. It wasn't fast but it you could get what you needed to do done. But most tasks came in via terminals at 9.6kbps so it so it never was quick to begin with.