I doubled this recipe and used an 8 serving box of jello (but the sugar free kind so the weight is different). I had a hard time determining when to add the cool-whip. I tried to take acerst's advice and wait until the jello was starting to gell but I think I put it in too early. I also just used a whisk to mix it but mine had two distinct layers. One that was light pink and had cool-whip in it, and one that was just jello. I enjoyed it as I am trying to curb my calories right now. The kids were not fans though. Thanks for a good one to try!
Tried this with Strawberry gelatin and chilled it in tart shells garnished with whipped cream and strawberries. It worked very well and sat up nicely. I added the Cool Whip to the gelatin just after the gelatin had begun to congeal around the edges (about 1 hour). The gelatin/Cool Whip mixture began to set up immediately after they were combined but a quick stir made it possible for me to still pour it into the tart shells and make a smooth filling. I think I'll try this next with a ladyfinger lined springform pan and use lemon gelatin. Yum! Thanks for an easy, wonderful and versatile recipe!!