They were not isolated individuals, struggling alone to follow Jesus. They were a new community that invested heavily in relationships by giving their time and money. When you see the word heart in the New Testament it is referring to the seat of physical life, the seat of moral nature and spiritual life. The heart represents the true character of a person. The NIV translates soul as mind.
Soul is the more accurate word.
Soul denotes the breath of life of the body. They moved as a single entity. They throbbed as one heart. They were all moving in the same direction with similar objectives and values. When you make an economic investment in another person's life, you have deposited your heart with them.