We did it one year fairly successfully at u16, the B team was to be a feeder team to the A team. Rostered 15 players with A team and 18 with the B team. There were 5-6 players that moved back and forth weekly, just about all players got some game time with the A team, especially during summer tournaments. Summer training was together, multiple coaches, would separate defense, midfield, forwards for drills. Fitness and scrimmaging was mixed. Once the Fall season began the first third of practice was fitness and drills together, then individual team work players were told weekly which team they were playing on for the weekend. Tier 1 team finished middle of bracket and Tier 2 team won their bracket.
It was working pretty good until the parents started bitching. People complained at the top because they didn't win the bracket, people in the middle complained when they thought they should always be at the top and people on bottom complained because the top team always took their best players. Coaches got tired of the complaining went back to 2 teams, full rosters, practicing on different nights, very little movement between teams unless someone was short handed.
Maybe I liked it because my kid was a legitimate tweener and I was realistic about his ability. He could be on the bench with the A team or start on the B team. It was nice to do both. He decided he would rather be a starter with the B team when the experiment ended. Both teams played Tier 1 at u17/19. Neither ended up being especially successful, A team always above middle bracket and B always below the middle. Maybe I liked it because it was the only time in 10+ years I felt a club was legitimately working like a club and not a collection of individual teams.