One of the most important things for you to consider is the quantity and quality of key moments in each section of the entire story. If there aren't enough and/or those which are there don't feel potent enough, then you're story could very well seem stretched in places.
A few risks to remember:
- Can your supporting cast be fleshed out substantially without overcomplicating the main plot?
- When you do introduce new minor characters or expand on existing ones, does it seem like they might pick up too much attention from a reader?
- Do you find yourself having to review your main characters and their pivotal events to adjust, then readjust, and readjust some more (i.e. making major changes to the story as you mentioned)? If so, you could easily change the story fundamentally from what you originally envisioned.
Assuming you have all your risks under control, you can then focus on trying to expand where you can:
- As it is crucial for the reader to connect, just try to add more to what exists now until you're satisfied that is likely.
- If you then feel as though what you have is too big for 2, then try to look for holes in both your primary characters' and minor characters' story for places where you can slip in some extra drama/backstory which can serve to pull the reader into the world (this is especially effective for fantasy), give the reader more to consider when they're trying to guess who's good/evil/will become important etc. or to set up or improve good plot twists.
You might pick a character, who originally would not have been much more than an extra in a movie, and have them seek revenge on the main character because of some complicated or misunderstood event in the past. This would serve to break up the main character's scenes and allow the viewer to explore more people in the world, and perhaps even establish perspective.
Ultimately, you can either go with upping the tempo of the story so you can slice and dice what isn't necessary, or you can go for more in-depth tale, in which case you'll definitely want to add more serious tones, obstacles and tangents to enhance the story.
I can only go on what you've given so... if you're going to kill off that main character and his 'friend' in the first book, then you'll need to give them, or the people who talk about/whose actions define those primary characters, a lot of time and situations which reveal their nature. Try to ensure a person who isn't familiar with your characters can say back to you something at least close to the character bios you may have made for them. If they can, then you obviously got what you needed into the first book.
As for delaying the resolution of the novel, that's fine, as long as you indeed get the reader to connect to the characters.