@H. Malcom WalkerIt's not that it's 'jarring', as 1st-person is simply a more limiting perspective. Technically, since the protagonist IS the narrator, the narrator can only describe what he's personally experienced.
There are many workarounds for that (ex: having other characters recount what they others have told them), yet a better option is to try the 1st-person alternatives (ex: 1st-person Omni, 3rd-person Limited and a few other alternatives, as that way you can handily dance between the various limitations). Unfortunately, that those each take time to learn and then master, still they often the best of both.
But as I've often said, the main problem with multiple perspectives, is the reader has no one to focus on, and too often, if there are too many protagonists then the reader has no one to follow, as readers love living live vicariously as the protagonist. And if they don't, they're more likely to not care whether either one actually survives.
So it's simply a more precarious story to write, as you're actually making it harder for you to succeed. It can be done, yet is IS more difficult, a higher hurdle to clear with each lap, as it were.
Again, those other perspectives are better handled via other means, (ex: an Omni-view, in whatever perspective, as then you can easily switch between the characters, yet still maintain a clear protagonist and an independent narrator to describe what the protagonist can't know, as the narrators role is to describe what happens to and around the characters, thus they can better describe the overall story, making the whole story arc easier to maintain.