Caballero was reunited with Pellegrini in July - the 32-year-old goalkeeper previously played under the Chilean at Malaga - as City prepare to launch their Premier League title defence.
Hart has failed to start just seven league games in the past four seasons, but was dropped last season following a string of high-profile mistakes.
The 27-year-old, though, quickly established himself as the team's first-choice stopper and Pellegrini has praised the competition that Caballero has added to his squad ahead of the season curtain-raiser as City face Arsenal in Sunday's Community Shield.
"I have two number one goalkeepers," said Pellegrini.
"I am sure Willy will be very important for us but I also continue thinking we have the best goalkeeper in England in Joe Hart.
"Top teams need two players for each position because we have to play so many games during the year."
