Why would the partnership be more unbalance than the common two right footers?
Do you have any data on this at all, seems like a bizarre theory to me. Also I don't get your point on Fred? He's a left footer and we have McT who plays on the right, why wouldn't he be on the left? We don't have anyone that would sit behind them, irrelevant to what foot players use.
I don't understand what you mean by the bolded part, but I'm posting on the assumption that we would bring in Camavinga to pair with Fred. You already think it is a bizarre theory, so trying to explain this would be a bit awkward. I'd like to provide some data, but I don't see this as something that someone would make a data for.
It is about player's stronger and weaker sides. There are exceptions, and players could easily adapt, but there are generally more right-footed footballers, and in the midfield areas, the team without the ball are more inclined to protect the middle.
A player's stronger side being on the opposition's weaker side or the side that the opponent is less inclined to protect is a favorable match-up. It is natural, and makes it easier to retain possession against the press. When the middle is crowded out, you fall back to the advantage of moving the ball to your stronger side against the opposite players weaker side.
A left-footed player playing on the left side of a midfield two - like Fred and Matic does for us when paired with McTominay, has the advantage of having their stronger side away from the middle. A right-footed player playing on the left side of a midfield two has the advantage of having his stronger side on the opposite player's weaker side. A left-footed midfielder playing on the right side of a midfield two - as would be the case with two lefties - is disadvantaged on both counts, which is my point. It is not rigid, and there are exceptions, but the small advantages matter.
It is all theories based on my experience and understanding of football, and I wish I could provide some data to back it up, but one will never understand how uncomfortable it is to play on the right of a midfield two compared to the other side as a midfielder who favours the left side, until they experience it at a decent level and are unable to adapt. The zones the right-sided midfielder in a two is naturally expected to occupy is not favourable for a left-footer.
Have you ever thought about why a partnership of two left-footed CBs rarely lasts, or why Nemanja Matic avoids the right side of the pitch completely when he has the ball?