Usually the only benefit to using a multi-part request (if supported) is that it saves the latency of the round trip from multiple requests. The server still has to do the same amount of work. In general, I'm not aware of such multi-part request support in ELM REST APIs. If network latency is an issue, you can always perform multiple requests concurrently. However, be careful to limit the maximum number of concurrent requests because it can result in the server's connection pool being exhausted, to the detriment of other users.

In some cases, an OSLC query or a reportable REST call might be able to query for and return information about multiple objects. You'd have to look at the API documentation to see whether that was possible for your specific use case.