I think the way you worded the question is misleading. It looks like your asking whether or not you need both Raikou and Entei for Suicune to appear, in which case the answer would be yes. You need both Legendary Beasts for Suicune to appear.
I think your asking, however, if you need to catch both Entei and Raikou, in which case the answer would be no. Since only one can be caught depending on what version you're playing, you have to trade for the other version. To encounter Suicune, you need both Raikou and Entei in your party, and neither have to be caught in your game.
Hope I could help!