Like LaRue has already mentioned, you want to train your elderly clients the same way (with appropriate modifications, of course) you train other clients. With doing only 2 sessions per month, I would think about having your sessions be more consultative and educational rather than a regular workout.
You can have your clients record their improvements as they complete their workouts during the times you are not training with them. You can review their progress during your sessions, add a few new exercises and make corrections as appropriate. On a seasonal basis you could complete your regular fitness assessment to measure their overall progress and change in health status.