I find that it can be confusing to talk about applications in “mixed” Taiji company. Taijiquan has “postures,” “energy techniques,” “palm methods,” “usages,” etc. All these terms cover different territory. Different people use the word “application” to cover different concepts.
I would argue that traditional Yang Style sometimes groups more than one fighting possibility within a posture, so that some postures are practiced in a way that does not correspond to anything one would do in a fighting situation. It is not hard to isolate single Whip into two different types of strikes, but I would agree that it is hard to understand it in the context of simultaneous attacks by two people.
One way to view Single Whip, which we have discussed in the past, is most easily seen out of Cloud Hands. After using your right arm to ward off the opponent’s strike upward, imagine using the Tiger’s Mouth of your right hand to pull the opponent’s right arm over your left arm and across your body. Then twist the opponent’s right arm so that his elbow comes toward your chin and upward. Hold the arm outstretched with your own arm out straight. Then step behind the opponent’s right leg and use your left arm to ward off the opponent’s body across your left thigh. Use An (Push) energy to finish him or her off, or to strike if you happen to miss the takedown.
A Chen Style variation of the above is to imagine the following. The opponent uses his or her left hand to try a palm strike against your upper body. You then use your right hand to sweep the palm strike to the right and to grab and twist the opponent’s fingers rightward and outward. You then stretch out the opponent’s arm by extending your arm out to the right, with your right fingertips pointing leftward.
I hope this helps.