I'm not sure there is a GraphQL endpoint in core Magento that allows you to define multiple shipping addresses on a given cart. The core setShippingAddressesOnCart
mutation does NOT allow multiple addresses, confirmed in the resolver class \Magento\QuoteGraphQl\Model\Cart\SetShippingAddressesOnCart::execute
if (count($shippingAddressesInput) > 1) {
throw new GraphQlInputException(
__('You cannot specify multiple shipping addresses.')
);
}
Looks like this feature was dropped by the Magento dev team due to low priority. I can see that a PR was raised for GraphQL multi-shipping however:
Looks like you will have to build this resolver yourself, the PR linked above should serve as a solid starting point.