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You can easily include hidden products in your URL for adding to the cart. If you built an extension and you know which products where always associated with the product, it would be easy to add all the products to the cart in the background.
In a sense I am adding multiple products to the cart through one call here
We have developed a module which extends standard magento’s attributes/options functionality and allows to setup flexible configurator based on rules which can be managed by admin. Also it allows your customers customize product colors/textures.