Thank you for this insightful answer, Eric B.While NF and SP appear to have nothing in common according to Keirsey's temperament matrix; Linda Berens introduced a "cross-factor" that ties them together, and explains what you describe.
Both are "motive-focused", which basically is a people focus as opposed to task focus. They like to work with others rather than force them into a structure. So yes, they will seem more "fun" to each other than the more serious NT's and SJ's.
Meanwhile, of course, what you will have in common with the NT's is the iNtuition, and you will differ on T/F (impersonal vs personal and emotional).
NT's and SJ's will sync more in maintaining structures; though they will disagree as to whether the structures are concrete (organizations, etc) or abstract (plans, ideas).