Suggestions

In my opinion, there are several types of independence: the kind you can develop by playing an accompanied melody, or when the melody passes between two hands, where the transition should not be felt.

For the first case, any Chopin piece is fine; for the second case, Bach.

marco56 wrote:

how do you create true hand independence? I practice hands separately and the piece flows smoothly, but when I join the hands together I cannot control the independence, and I am making no progress; I have read that it is a mental factor, are there exercises for this problem?

It is indeed a mental issue (whereas finger independence is more closely related to purely physical factors). I suggest you try, while playing hands separately, to sing what the other hand should be playing, but without "cheating," meaning without settling for less. This exercise should force you to concentrate your mind on something different from what the playing hand is doing.

Combination, I happened to discuss this not long ago with a neurophysiatrist friend who explained to me, in essence, what Carlos said. In this specific case, we were discussing jazz drummers and the independence of the "four limbs" (limbs). But the meaning was the same and even the suggested exercises were the same: singing (that is, thinking), before performing, a completely different part, in order to make the rest "automatic." I don't think it's a very different discussion.

marco56 wrote:

how do you create true hand independence? I practice hands separately and the piece flows smoothly, but when I join the hands I cannot control the independence

When joining both hands, do you keep the speed unchanged and attempt to perform as if they were separate hands... or do you lower the metronome setting?