Can I use multiple first-person POVs?
Yes. Of course, you can use multiple first-person POVs... you just do it...
But how do you do it?
The biggest problem with multiple "I"'s in the story is going to be reader confusion. How do you make sure the reader knows who's narrating on every page, even in every sentence?
What I've seen of multiple first-person narratives, is that they seem to be divided up into clearly separated sections of the story.
For instance, the same event is told by several people. Each narrator gets a section and the previous section is finished before the next one is started, so they never "bleed together" in the text. (An example, not a book, but the principle is similar, would be season 2 episode 11 of Colony, "Lost Boy").
Or otherwise use separated sections but not retelling the same story ("Cloud Atlas" for instance).
One "kind of" example is One Thousand and One Nights. There is one narrator telling stories in third-person POV (Scheherazade). Sometimes in these stories, a character becomes a narrator in another, inner, story (a story in the story... in the story...) You might get ideas from how the transfer of narrative between these stories work.
Another common variant of divided-up stories is the short story collection. Being a collection of short stories, it's definitely ok to use multiple first-person narratives. (You might want to check out "The Decameron"—I've likely read it in school, but I don't remember exactly how Boccaccio deals with several characters hidden away in the countryside... maybe you could deal with your travelers' anecdotes in a similar way...)
If you don't divide it up, you need to make it possible for a reader to define whose narrative they are reading (for instance after picking the book up after a pause).
Use such elements as:
- Narrative voice
- Plot and problems—if one narrator is having trouble at work and the other is hunted by the mafia, their plots and problems could identify who was who.
- Other characters present only in one narrative
- Locations only used in one narrative
- Genre (Cloud Atlas)
- Style (Cloud Atlas)—for instance, each narrative comes in different styles (letters, e-mails, diaries, blogs, chats, etc.)
Especially the voice can be effective in identifying who's narrating, and it should still be unique regardless of if you use multiple first-person or third-person narratives, or even if the character is a narrator or not.
Things you should try to avoid:
- Switching narrator in the same scene or chapter
- Relying solely on scene or chapter headings to inform the reader who the narrator is
Will it work to use multiple first-person POVs?
To figure out if your multiple first-person POVs work you can do one or several of:
- Use beta readers to tell you if they got confused/liked it or not/etc.
- Put the first draft away for a few months and then read it. If you wrote something confusing or otherwise bad, you'll likely notice.
- Have an editor look at it (likely a developmental edit or evaluation), they should be able to tell you if it works or not.
Is it common to use multiple first-person POVs and should I do it?
No, it's not common to use multiple first-person POVs in the same story, but it does happen.
If you should do it or not is hard to say. If it's your first story, I'd recommend against it. It's not as easy as doing a single first-person POV.
In your example, maybe you could do Larry's anecdote from Bob's POV? It would be a different perspective on anecdotes and it could help the overall impression by adding variety.
If, however, you decide to use two first-person narrators, then yes, by all means, introduce the concept early on, in order to not make Larry's scene seem an odd man out. You may even want to look into using Larry's POV even more often, even every second chapter, and that way establish a system to assist in figuring out who's narrating.
Transform it into third-person
One way to go is to transform the perspective into a third-person POV.
Writing a scene in first-person POV and then transforming it into third-person POV is also a great way to understand how to write deep, focused third-person POV scenes. Doing it for a whole novel would just mean more work, but likely also higher quality.