Barcelona forward Messi broke Gerd Muller's long-standing goal-scoring record of 85 goals for club and country in one calendar year with his double against Betis last weekend, while Ronaldo helped Madrid to the Liga title last term with 46 goals in 38 appearances.
"As I think the boss (Real coach Jose Mourinho) said after the Barcelona game, we are talking about players who are from a different galaxy and each of them is feeding off the other," Karanka said at a press conference.
"One beats a record and the next year the other one goes one better. Their respective clubs are fortunate to be able to enjoy their talent and exploit it to the utmost."
Ronaldo and Messi have both been nominated for the 2012 FIFA Ballon d'Or, with Barca midfielder Andres Iniesta being the third candidate to win the prestigious individual award.