Ours has got factory windows, so opening both sides, and we use both. Although that’s possibly because the layout’s conventional with stove on the driver’s side, so we open that window (and the rooflight) when cooking (stove lid/heatshield stops the flame blowing out). And the passenger side at night for ventilation (although usually the sliding door’s not quite shut, partially for ventilation but mostly because closing it properly tends to wake our youngest up 🙂 ). We’ll also drive with them both open slightly if it’s hot – gets really warm in the back otherwise.
Um. Not sure that answers the question 🙂 I think ideally both, if you want to save the money have an opener on the passenger side to open into the awning. Maybe. Problem is that until you’ve used the van you won’t really know, although given how much you must already have spent on the van and the kit, the extra for two openers is relatively small beer 🙂