I need to measure the distance between two moving marks on a silicone rubber specimen while the specimen is being pulled apart until it breaks.

To picture what I need to do, imagine two objects 3 m apart.  They both start travelling in the same direction at the same time, but the one in front is travelling at a much greater speed and the difference in speed changes with time. The task for Tracker is to measure the distance between the two moviing objects.

I've managed to get Tracker to track each mark individually, but not at the same time (otherwise I could have taken the difference between the x or y values of each mark). The online manual for tracker suggests that I can set up a "tape" and attach each end of the tape to each object/marker.

But I have not been able to get tracker to attach both ends of the tape to separate moving targets and autotrack the distance between the two moving targets

Has anyone tried to do something similar with Tracker? Do you have any hints?  Do you know of any video tutorials showing how to do this?