Today's example isn't subtle, but it is old. And it would have been on the newsstand today in 1941.
Blue Ribbon Comics v1 #9 (February 1941) [M.L.J. Magazines Inc.]
In the Corporal Collins Infantryman story in this issue, the lead character runs into an old friend of his, Sergeant Boyle, who just happens to have a comic strip in another MLJ comic book, Pep Comics. Their side kicks, Slappsie and Twerp also appear. As becomes typical in character crossovers, they get into a fight and then team up to defeat the Nazis. When they meet:
Expect more of these in the future.