When someone makes a bun out of two frogs belly to belly, then puts their meat inside essentially creating a burger.
"Dude I heard rodney did a frog burger last night man!" - "No way dude that's insane."
When Rodney gets two frogs and lays them belly to belly, then fucks in between them essentially creating a burger with the meat patty being his dick and the bun being the two frogs.
"Rodney had a frog burger last night, some would call him a frog fucker." ... "Ever heard of a frogney burger? It's basically Rodney and a couple of frogs having outercourse."