Yes, its possible.
Like you said, both altered is the best, to ensure the least amount ofaggression/territorial issues, but its worth a try. My friend has anunspayed female who gets along fine with my neutered boys.
Are you considering getting William a lady friend? If so, youcan always try bonding and if the female is too aggressive considerhaving her spayed later (its best to anyway for health reasons).
Hope this helps!
Haley