With an AB living trust (sometimes called an "exemption" or "bypass" trust), the property contributed to the trust by the first grantor to die will be distributed to his or her beneficiaries when the surviving grantor dies. The surviving grantor cannot change the beneficiaries of the deceased grantor's trust property.
Imagine that a husband and wife create a typical joint trust and the husband passes away. Under the LegalZoom Joint Living Trust (and many other joint trusts), some or all of the husband's share can be held in trust for the wife during her lifetime. During her lifetime, she can change who will receive that trust property after her death. This may be exactly what the husband wanted. On the other hand, the husband's wishes may have been, "I want my wife to be able to use my share as long as she lives. But after she dies, I want to make sure that whatever is left goes to my beneficiaries instead of to people she chooses after I'm gone." If you want to make sure that each person’s property goes to his or her separate beneficiaries, an AB trust may be a better choice than a joint trust or a simple will, regardless of the size of your estate.