We have done a lot of testing in order to make Sleep Cycle work with two persons in bed. The short answer is: yes, Sleep Cycle works very well with two persons in bed. It could be a problem if the person using Sleep Cycle weighs much less than the other person but in normal circumstances it works very well.
There are two cases regarding this issue:
1) Sleep Cycle has a hard time detecting your movement.
In this case the movement of the other person will not be registered at all so there really is no issue. Sleep Cycle will work in a mode that uses all movement detected for the sleep analysis.
2) Sleep Cycle detects a lot of movement, from you AND the other person.
In this case Sleep Cycle will adjust its algorithm to only register larger movements. The small movements will be filtered. Since the other person is further away from the device these movements are smaller and will be filtered out.
Our extensive testing show that this works well.