The OS should avoid scheduling two servers (or, for that matter, any two CPU-intensive processes) on a single core if there are underused ones available.
Servers will not restrict or anyhow mess with their affinity, although they could be doing that if you believe that you know your server behavior patterns better than the OS does (this is a plausible scenario as you have a big picture view while OS scheduler is making momentary decisions based on server’s dynamic behavior).