Working with ASP.NET Web API, the most unexpected thing is the limited support of POST data values to simple ApiController methods.
When a parameter has [FromBody], Web API uses the Content-Type header to select a formatter. At most one parameter is allowed to read from the message body. The reason for this rule is that the request body might be stored in a non-buffered stream that can only be read once.
A simple principle, you can send any content in HTTP request, it only need to be serializable into a string. So, it could be multiple JSON object. In this example, the content type is "application/json" and the request body is a raw JSON string (not a JSON object).
Here I found a workaround to pass multiple complex objects (using the above principle) from jquery to a WEB API using JObject, and then cast back to your required specific object type in api controller. This objects provides a concrete type specifically designed for working with JSON.