Hi Tricia ... thanks for asking the question on the forum. Perhaps others can share their experiences on how they have fused both yin and yang elements in their classes.
As always, it depends upon your intentions. I have often lead fusion classes. If my intention is to do some deep physical work, I will start with Yin first, while the muscles are still relatively cool. [See this section of YinSights for an explanation of why we do it that way: http://www.yinyoga.com/ys2_1.1_when_to_practice.php
and then read this page for the difference between doing yin when warm or cold: http://www.yinyoga.com/ys2_1.1.1_cold-warm_muscles.php
However, if my intention is to have a mental/mindfulness class where the priority is to really focus the mind, I may start with yang flows first and end with yin. I do this especially if the class is a later-in-the-day class. It is nice to end the evening with a yin-state of mind. Here in Vancouver, on Thursday nights, we have a "double-dip" schedule where I lead a Power/Vinyasa class from 5:45 to 7:00pm and then this is followed with a Yin Yoga class at 7:15pm for 75 minutes. Many students stay for both classes, and so end their day with the mellow Yin feeling.
Sometimes, I will ping-pong between yin and yang offerings in one class. Here I would generally start with the yin poses for maybe 10 mins, then move into yang, then end with yin before final meditation or shavasana. If you are going to weave together yin and yang elements, make sure each section of the class has at least 10 mins of time, so that you aren't bouncing back and forth between the two modalities.