One way to make the transition easier is to have more Points of View - chapters or interludes where the main character is not the POV. For example, if the main character tells someone that they are going to bed, and will see them in the morning, you can follow the other character overnight, and hand the POV back to the main character when they meet up again. It needn't be long - just a paragraph or two at a time - but it will help the reader to expect POV changes.
This has 2 additional effects: first, the Primary Protagonist can carry out actions that the reader hasn't seen (be sure to add hints though, so that the reader can work out afterwards what they did when, and maybe even work it out before the reveal) and, secondly, it will initially obscure that the new character is going to be a Secondary Protagonist; the reader is used to diverting to different POVs, so won't realise the character's importance until they notice how lengthy or frequent their POV sections are.
As an example of this type of hand-off, you could consider the first chapter of "Harry Potter and the Philosopher's stone" (Or, "Harry Potter and the Sorcerer's Stone", for
Philistines Americans). The chapter starts with Vernon Dursley's POV, swaps with a clear transition to Professor McGonagall's POV, and then blurs ambiguously into Professor Dumbledore's POV at some point for a brief moment, before jumping to Harry himself for the final paragraph.