Not all IEX integrations allow the setting up of multiple sites for the same e-conomic agreement.
Note that if the same item numbers/SKUs are used across web shops, products will be created with the data / language specified on its site of origin and that when a product is updated, it will be inherit the data / language of the site the product from which it was last saved.
Therefore, it may be a good practice to always create the product on the same site each time, and then disable selected fields during product update in the IEX integration so that product data will not change unnecessarily between last saved data / language.
At time of writing, the only solution that supports multi-site integrations is WooCommerce.
The price of each extra site is € 7 ex. VAT per month. Contact IEX for a price overview if you wish to connect more than 4 sites.