There is no one definitive answer to this question as it depends on a number of factors, such as the type of ecommerce platform each site is using, whether or not the two sites are owned by the same company, and so on. However, in general, there are a few ways that you might be able to create a shared shopping cart between two ecommerce sites:
-If both sites are using the same ecommerce platform, there may be a way to enable a shared shopping cart through the platform's settings or by installing a plugin or extension. For example, if both sites are using WooCommerce, you can install the Storefront Shared Cart plugin (https://woocommerce.com/products/storefront-shared-cart/).
-If the two sites are not using the same ecommerce platform, you may still be able to create a shared shopping cart using a third-party service such as ShopSync (https://www.shopsync.io/).
-Another option would be to create a custom solution using some combination of programming languages, APIs, and webhooks. This would be a more complex solution and would require a fair amount of technical knowledge.