Xen - Make one guest OS have 2 cpus?

I believe you just add vcpus=2 to the guest's config file.

You need to add/modify the vcpus line in the guest's config, however you should be aware that some of the more recent versions of Xen manage the configuration via a backend buried somewhere in /var so you will require more than just a restart of the Guest for it to pick up the configuration changes. It may work fine with a simple restart of the xend service or a reboot. If that doesn't work, an xm destroy then xm create of the Guest should suffice.

(Usual caveat, back up your configs before something permanent like an xm destroy)