Would you inform both sides of the family once you and your spouse decide to split? How would you suggest to announce such a sensitive thing to your families? What should I take note of? Should I let him tell his and I inform my side, or should we do it together? Please help!

It is better to inform both sides of the family of your decision as to keep them aware of the decision. I suggest to announce it together as it is the joint-decision by both of you. It is good to also state the reasons what lead to the decision so the family members can understand better instead of blaming each other parties.