Hi everyone. And thank you for welcoming to the KSP forum.
I've had a chance to analyze the situation, and It's completely my own fault. Both vessels launched independently and rendezvoused in orbit. Or so I thought. After adjusting the course of both vessels to rendezvous, I managed to put them both on a re-entry trajectory with a Pa of 54.000. As the mission description clearly states, the vessels have to be in orbit.
After retrying the mission, I managed to complete the mission by accident with over 15 km of distance - but this time they were in orbit.
Have a great day or evening or whatever it is where you are