How could he have had contact with a second person if he was in quarantine?
There was probably a few days after meeting the first person before that person tested positive and Cruz started his quarantine.
Which gave time to meet the second person.
That he met two that are testing positive is an indication of how dangerous it is in those political circles.
It was a second person after the initial contact and before he self-quarantined - had to add some days to close the window.