I was thinking of some films where the combination of two main actors works perfectly. Here's some I came up with:
Clint Eastwood/Lee van Cleef in For a Few Dollars More
-This is obvious and probably the most legendary pair in spaghetti westerns.
Henry Fonda/Terence Hill in My Name is Nobody
-Hollywood legend meets new european star. This will end up in history books.
Franco Nero/Tomas Milian in Companeros
-This is one of my favorites. Two great stars of the genre and very different actors together. Still they work perfectly together never overshadowing each other.
Feel free to continue...