Hi,
The web services were built to service the UI. So if you don’t see something happen in the UI, it’s likely not doable via the API. That said, multiple projects are displayed together - with measures - on the projects page.
The best way to master the API is to perform the desired action via the UI and eavesdrop to see which calls the UI made to accomplish the action.
You may also find this guide helpful.
HTH,
Ann