I do a lot of pec stretches during chair massage and have never had that problem....I'll try to explain how.
The first one is not a great stretch, but it's a good start. I stand behind the client, one leg back to brace myself, reach over both shoulders and pull back, leaning my weight into my back leg. That stretches both at once and is helpful in warming the muscles for further stretching.
Another, very effective pec stretch, is to stand facing the client with my right arm to his/her right arm, so that I am sort of off to the side. With my right hand, hold the anterior shoulder and with my left hand hold the right lat. Lean into the shoulder and hold the stretch. The key is to feel the muscle and not go past the first barrier. Stretch TO the resistance, not beyond, and then hold there. Let the client know to communicate with you if there is any discomfort.
While you are doing this pec stretch, it is easy to get the other hand underneath the scapula and work out any TPs in the subscap. That will help free up the TO.