Personal view is that this would be massively complicated to manage, and would likely result in several problems. For example, City get promoted solely because of the PL loanees which are then withdrawn once they are promoted. City then struggle to survive and get relegated back to League 1...
But what would happen if a "twinned" team got promotion to the PL? The reason why this B team proposal keeps getting rejected is because it compromises both individual Club independence and the pyramid, and the pyramid is to my mind is one of the best things about football in England.
The interesting thing about this is that most agree on the problem - which is young talent not getting a chance to play at a suitably competitive level. But most can`t agree on the solution.
I think this is a problem created by the PL clubs themselves, who overdo the signing of quality...