Yes! Neither of these products can be used successfully without the other.
The Foundations Teacher's Manuals contain the actual lessons and instruction, as well as the directions for each activity in the workbook. The scripted lessons they provide are the core of the curriculum, guiding you through teaching the concepts. The Teacher's Manuals also include many teacher tips for teaching concepts successfully, multi-sensory fun, and other engaging activities to do for reinforcement and practice.
Many of the games and activities in the lessons do not use a workbook page. The Student Workbooks contain the student response portions for the hundreds of activities that do. These include both instruction activities and practice activities for reading, phonics, handwriting, and spelling concepts taught in the lessons.