How to Update Custom Column Value in Quote_Item Table – Magento 2

could you explain more your task ? the customer 1 has to keep his quote and items ? and the customer 2 just have the same items than the customer 1 that's it? In this case just use the addProduct method which take 2 parameters ($product, $qty).

Get the items from customer 1's quote. Loop on it, and add product to your customer 2's quote.

You can use event/observer to achieve this.

Use following codes in your module.

etc/frontend/events.xml

<?xml version="1.0" encoding="UTF-8"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="checkout_cart_add_product_complete">
        <observer name="bialcheckout_product_add_to_cart" instance="Vendor\Module\Observer\ProductAddToCart" />
    </event>
</config>

Observer/ProductAddToCart.php

<?php
namespace Vendor\Module\Observer;

use Magento\Checkout\Model\Session;
use Magento\Framework\Event\Observer;
use Magento\Framework\Event\ObserverInterface;
use Magento\Quote\Model\QuoteRepository;

class ProductAddToCart implements ObserverInterface
{
    public function __construct(
        Session $session,
        QuoteRepository $quoteRepository
    )
    {
        $this->session = $session;
        $this->quoteRepository = $quoteRepository;
    }

    public function execute(Observer $observer)
    {
        $quoteId = $this->session->getQuote()->getId();
        if ($quoteId) {
            $quote = $this->quoteRepository->get($quoteId);
            if (!$quote->getIsActive()) {
                return;
            }

        $product = $observer->getEvent()->getDataByKey('product');
        /** @var \Magento\Quote\Model\Quote\Item $item */
        $item = $this->session->getQuote()->getItemByProduct($product);
        $itemId = $item->getId();
        $quoteItem = $quote->getItemById($itemId);
        $customValue = 10;  // Prepare your custom field value here
        $quoteItem->setCustomField($customValue); // Set custom field value
        $quoteItem->save();
        }
        return $this;
    }
}