bed them at the same time, or at separately, most people don't have any convincing evidence either way, unless you're using a funky compound that may react with the cementite and other build up on the old rotors.

bed the SHIT out of them, meaning, get em nearly smoking, but not quite. the method i got from cobalt-friction is:

5 medium hard stops from 50 m/h to 10 m/h

10 minutes cooling

5 hard stops from 50 m/h to 10 m/h

cool overnight.

do not come to a full stop with the new pads as they may glaze if you do. it's best done in an unregulated, open space, or late at night on empty roads with few signals.