You are given a list containing multiple decks of cards, where each deck consists of 52 unique cards. The total number of cards, n, is guaranteed to be a multiple of 52. Each card is represented as a structure containing two fields: suit and rank. Write a function to shuffle all the cards together using a custom algorithm that mimics manual shuffling.

Constraints: The total number of cards, n, is guaranteed to be a multiple of 52, indicating that there are multiple decks. Each card has a suit and a rank. Suits are one of "Hearts", "Diamonds", "Clubs", "Spades". Ranks are one of "A", "2", "3", ..., "10", "J", "Q", "K".

Function Signature:

struct Card {
    std::string suit;
    std::string rank;
};

std::list<Card> shuffleAllDecks(const std::list<Card>& all_decks);

Input: The input all_decks is a std::list of Card structures containing one or more decks of cards. All the decks are combined into a single list for shuffling. Output: Return a std::list<Card> where all the cards from the input have been shuffled together.

Example:

Input:
std::list<Card> all_decks = {
    {"Hearts", "A"}, {"Hearts", "2"}, {"Hearts", "3"}, ..., {"Spades", "K"},
    {"Hearts", "A"}, {"Hearts", "2"}, {"Hearts", "3"}, ..., {"Spades", "K"}
};

Output:
std::list<Card> shuffledDecks = shuffleAllDecks(all_decks);
// Example output (shuffled order):
// [
//  {"Diamonds", "J"}, {"Hearts", "3"}, {"Clubs", "A"}, ..., {"Hearts", "4"}
// ]

Explanation: The function should simulate the shuffling of all cards combined by thoroughly mixing the cards from all decks together, mimicking the randomness of manual shuffling.