A double stroller folds wider and heavier than a single, so a padded travel bag and a bit more care around the fold lock go a long way.
Pro tip: If the double stroller separates into a base and add-on seat, pack them as two smaller pieces rather than one bulky fold.
Watch out: Double strollers are wider and heavier than single strollers, do not let the folded unit's wheels rest directly against upholstered furniture during transport.