Yes - and the flip side of that is, "don't put two full encounters in adjacent rooms and expect the party to prevent room 1 from warning room 2." Speaking from experience, that's a TPK recipe when room 2 piles into room 1 before the party have had a decent chance to spend healing surges, recover Encounter powers, etc.A good trick when converting older editions to 4e, imho, is remembering to put multiple rooms together as one encounter.
If that's the kind of feel you want for an encounter, you want a Level +1 or Level +2 or even Level +3 encounter spread out over 2 or more rooms ... if the party can split off the first room and keep it from getting the warning out, you have two easy encounters .. but more than likely you get one challenging encounter with success depending on how well they handle the first group, how well they adapt to the "reinforcements" arriving, and how they use the terrain to their advantage.