We don't currently have plans to allow multiple schedules to be selected, but we do have another feature intended for the Trend Analysis report called Item Groups that should prove useful.
Item Groups basically allow users to set-up a single reportable 'item group' which can include multiple items across multiple schedules. It could also be used to create a single item group which combines multiple schedules.
The Item Groups feature is actually already a part of Front Desk, but for now it is only available on a select few reports such as the Billings Report. We intend to expand this feature to cover more reports in the future, including the Trend Analysis report.