Using REST API and migrating to the new one as old is been deprecated.
We have products that have more than 1 box (2,3 or 4). We will like to add more than 1 tracking numbers to the same product, but seems like the REST API fulfillments.json does not allow it. I read something about a possible update.
Well you could use the GraphQL mutation to do it instead. Its not particularly difficult. We migrated all of our fulfillments to graphql when we migrated to the new fulfillmentorder api.
There used to be a way to do it. just by passing an array of tracking ids and tracking urls. Maybe its undocumented. Give it a try on a test shop and see if it works. GraphQL does have a bit of a learning curve but it has some very very powerful features not available in the REST apis (like batch updating 2300 products inventory in under a minute).