There is probably an easier way to do it in ETABs, but what I've done in the past is model the beam at one floor, then draw in small members to link it to the floor below.
My main goal being to make sure the gravity load from both floors makes its way into the beam. Not really worried about torsion / twist, or such.
You can model the beam as a spandrel beam (wall section, then assign a spandrel label to it) which requires you defining the two floors in two different levels.
Or, you can just model them as one floor at same level as Gaurav Malya said. You may try both and let us know which worked the best for you!