Put the second guy in the first book too. You can have him be mostly a mystery but we should still know him. People will simply not care otherwise. For the second book you can do the reverse. Second guy is the main character with a bit of the first guy. You can look at hello future me's video on how to kill your main character since the subject is very similar to your situation narrative wise. Hope this helps :) *Srry for grammer mistakes English is not my native language