You may manually bind tracked devices to bones of the skeleton before the calibration if you want to use two trackers to track one bone or if binding of some trackers failed during calibration.
Select the active skeletal mesh, then create new calibration preset or choose already existing one from the list.
Open the collapsed list Setup Optional Bindings and press “Update” button on the right. The software will display a list of active Vive Trackers with their IDs in the SteamVR system.
Click on a button with the tracker’s ID to highlight it on the screen to the right.
Choose bone from the drop-down list for the trackers that you want to be associated with this bone. The same bone can not be associated to more than two devices.