Many "running backs" also incorporate one or two stages of checkstays and so would be impossible to tack without releasing one and setting the new one.
This is done during the tack - the runner that is on (to windward) is held as the tack is begun, then released as the boat goes thru head-to-wind. As the rig loads up on the new tack the (formerly leeward) runner is tensioned as required as the boat comes up to speed. The new "lazy" runner to leeward can be loosely set up on its winch or tackle ready to be tensioned on the next manuoever.
With today's large roach mains, even single part running backs can hang up the sail if both are left "on"
Downwind you need to keep an eye on the lazy runner so that it doesn't end up draped on the wrong side of the main, but handling them is similar - swap them mid-gybe. Forgetting to release a runner during a windy gybe can make life interesting.
Keep in mind that on many rigs runners may be more important when beating than running unless it's really honking out there. But it depends on the rig - some will fall down if the runners are mishandled.
Knowing what boat and the rest of the rig configuration would be helpful.