Then another family member can give them a compliment for that. Next, give family members a week to find something good about each family member so they can express it at the next family meeting. With patience and practice, this becomes second nature to children. Giving and receiving compliments is a great life skill. This is really no different in blended families than in any family.
Opposites do attract.
There is one difference that usually doesn't show up until children come along. That difference is that one parent is usually a little too strict, and the other is a little too lenient. They then fight about who is right and who is wrong when they are both wrong. It is not effective to be too strict or too permissive. They key seems to be to take a parenting class together so both can learn the long range results of what they do.