Forget one.
This is not true for the second wave. Occasionally, three possessions can happen between the death of the first boss and the end of the second wave, for a total of five possessions. From my observation, this happens regardless of whether the third (in total) possession occurs during the interlude or after the actual start of wave two.
Fortunately, this fifth possession usually occurs with less than 10s on the wave timer, so it’s rather safe to ignore it and just kill the second boss.