Hi Saske,
Here are the steps:
1. Create the first ROI (plus symbol in Track filtering) – you should see a sphere in the surface view and circles in the volume view.
2. Go to the volume view, press ctrl (command for mac) and move the circl to desired location.
3. Add another ROI – you should see sphere of different color. Select it by clicking on the ROI description in the track filtering menu. This means you have selected the desired ROI.
4. Go to volume view and move it around. You should see tracks that pass through both spheres.
Here is a ppt from an earlier training event that explains how to create spherical ROIs in section 3.3.
Link – http://brainsuite.org/wp-content/uploads/2015/10/Part3b_Bhushan_BDP_workshop_hands_on.pdf