Matthew McCallum wrote:
Prior to Spectacular Spider-Man, were there any other Marvel characters who had two on-going monthly solo books? If not, that might be a milestone to be recognized, although I would expect that to be Volume 250.

In terms of the Silver Age heroes, Nick Fury comes to mind since he was clearly the lead in two series, but the classic SHIELD material will wrap soon and Sgt. Fury has of course already started.