You can add multiple items on Service Requests as long as those items are not marked as a Loaner item in the Service Catalog.
You also cannot setup Loaner items in the Service Catalog that have additional items that the user can request.
In our situation we have a set of Laptops for checkout but we also have USB Monitors that they can checkout. It would be nice to be able to have extra items like that on a single request jut like you would if you are permanently assigning the user to a device.
Is there any particular reason that loaner items do not provide the same service request features as permanent items?