Two vs one?? Well, it depends on personal preference to a large extent. As you say, two makes it heavier and bulkier as well as more expensive. However, if you don't want to mess around changing visors for different flight conditions, then maybe it's worth two. For example, going from a dark lense to amber for lower light. Dual's can be bulky, but it depends on the helmet. The HGU-56 is bulkier than the HGU-84 no matter if it's single or dual. Some guys like the bungee-on visors too as they can be removed easily.
Keep asking around and decide what's best for you is probably the best way.