Горе на двоих - полгоря, радость на двоих - две радости

It is easier to bear trouble if someone shares it with you, and you feel happier if someone shares your joy. See Горе на двоих - полгоря (Г), Разделенная радость - двойная радость (P)

Cf: Happiness is not perfect until it is shared (Am.). Shared joys are doubled; shared sorrows are halved (Am.). Shared pleasures are doubled; shared griefs are halved (Am.). A sorrow shared is half a trouble, /but/ a joy that is shared is a joy made double (Br.)