By this I mean two people you could effortlessly swap in for each other in the majority of their roles and many people might not even notice. Another thread mentioned Dead Calm, a film where I mistakenly thought for years that the villian was Ray Liotta and not Billy Zane. Another one would be Anthony Hopkins and Michael Caine, though they’re different enough in that you want Hopkins for the edgiest roles, and Caine for when a comedic touch is needed-but they’d still be interchangeable for most of their roles.
There probably is more female ones tho (at the risk of sounding sexist, so donning flame retardant suit).