The Gospel According to John and the Johannine Letters (15 sessions) can become two, or even three, shorter studies. Here are some natural breaks in the material:
John 1 - 12, known as the "book of signs" Plan for Introduction + Lessons 1-7 (8 sessions)
John 13 - 21, known as the "book of glory" Plan to use Lessons 8-11 (4 sessions)
1, 2 & 3 John, the letters Plan to use Lessons 12-14 (3 sessions)
The Gospel According to Luke (12 sessions) also has some nice breaks which could create two studies with a break in between.
Luke 1:1 - 9:50, early life and ministry of Jesus Plan for Introduction + Lessons 1-5 (6 sessions)
Luke 9:51 - 24:53, travel narrative, passion & resurrection Plan to use Lessons 6-11 (6 sessions)
Letters from Prison (10 sessions) plus bonus section on 1 & 2 Thessalonians can be divided easily into shorter pieces with breaks in between.
The Letter to the Philippians Plan for Introduction + Lessons 1-3 (4 sessions)
The Letter to the Colossians Plan to use Lessons 4-6 (3 sessions)
The Letter to the Ephesians Plan to use Lessons 7-9 (3 sessions)
The First & Second Letter to the Thessalonians Use bonus section (3 Sessions)
Galatians and Romans (13 sessions) could be simply divided between the two Pauline letters.
The Letter to the Galatians Plan for Introduction + Lessons 1-3 (4 sessions)
The Letter to the Romans Plan to use Lessons 4-12 (9 sessions)
Voices in the Desert: Amos, Hosea and Micah (10 sessions) can easily become three short studies, each focused on one of the prophets.
Book of Amos Plan for Introduction + Lessons 1-3 (4 sessions)
Book of Hosea Plan to use Lessons 4-7 (4 sessions)
Book of Micah Plan to use Lessons 8-9 (2 sessions)
Women in the Old Testament (10 sessions) can be divided according to the way the women are grouped in the study set.
Women of Israel's Beginnings and Women of Israel's Passover Plan for Introduction + Lessons 1-3 (4 sessions)
Women of Israel's Early Tribes and Monarchy Plan to use Lessons 4-6 (3 sessions)
Women who teach us about God and ourselves Plan to use Lessons 7-9 (3 sessions)