If it's someone else that used an offensive technique and defensive technique at the same time, they would surely suffer tremendously. After all, the qi contained by the two techniques were very much the same, but they turned out to be different from each other. Thus, it could cause a contradiction between the two and it would affect the one who cast the techniques. In fact, it could harm future prospects and the talent of an individual. So unless there were no other choices to choose, no one would bother about doing this.
Jin Rou had already prepared for the worst outcome as he used two techniques at the same time just to clash with each other. He thought that he would, at worst, became immobile and suffered severely. But it seemed that lady luck was with Jin Rou as it only depleted his qi and was very exhausted.