Would It Work...

[Frontier]

Whereas you can have Miles as a teenage Spider-Man and Peter as the grownup adult Spider-Man and you can have a contrast and dynamic between them that illustrates some differences and similarities. In the case of Miles, Peter can see Miles and see both a success (his legacy as Spider-Man is meaningful and found value in the generation directly succeeding his) but also a kind of failure (since obviously Peter's not going to be the last guy with his powers and his responsibility which he would feel is some kind of failure on his part). So there's a poignancy in their interactions. The Spider-Man PS4 game captures that well, that mixed emotions. Right now, writers haven't tapped into that very well but it's there for the taking.

I think they haven't tapped into that well because they'd rather explore the characters in isolation then deal with them in a shared capacity, thus making the other Spider-Man irrelevant to the individual Spider-Man's books.

The only way you can really explore that in-depth is in a shared setting like the PS4 game, and even that had to really re-work Miles to make the dynamic work in my opinion.

[Digifiend]

It worked in the Ultimate Universe but it won't work in the 616 for long. Five to Ten years max if Marvel decided to go that route for some reason. And, why?

Miles is Spider-Man in his own solo and the Champions book.
Peter is Spider-Man in Amazing, Friendly Neighborhood, Symbiote, Spider-Man/Deadpool, various video game titles, War of the Realms, and other team-up books. He sells!
Gwen Stacy is Ghost Spider in her solo book.
Silk is in the Agents of Atlas book.
Jessica Drew is Spider-Woman in the upcoming Strikeforce book.
Eddie Brock is Venom in his solo book and the Savage Avengers.
The Absolute Carnage event has numerous symbiotes or ex-symbiotes.
And last but not least, Otto Octavius is the Superior Spider-Man in his solo book.

That's a lot of books being sold at once and it's f'ing great! Why mess that up just so Miles can be the main Spider-Man?

Spider-Man/Deadpool is over. And Silk joined it about half way through the run (when Robbie Thompson took over as writer).

[Ultimate Captain America]

There is a way to do it that everybody will love. Peter Parker and Mary Jane get married, and Peter leaves Miles in charge while he leaves to a well deserved honeymoon.
* Fans of Miles can read him as the main Spider-Man again
* Fans of Parker can read him in his honeymoon. It worked so well with Gambit and Rogue, a "Spider-Man & Mary Jane" book would be the bomb!
* This set up implies that things are not permanent. Meaning, it can continued indefinitely or until readers want Peter back, whatever happens first.