From ReactOS

One potential idea for the Session Manager is to utilise Intel's HyperThreading teachnology (running 1 session on each virtual processor), and/or supporting Both Core in an AMD/Intel Dual Core processor.

I mean, let's say we have a Dual-Core Intel Processor, with HyperThreading enabled. That's 4 effective processors, meaning you can run a Win32 Session, a POSIX Session, an OS/2 Session, and a DOS Session, without a noticeable drop in system performance.