Let me phrase it this way, I did my first ever race last fall, 24 Hours of Moab, and there is no way I will ever race with only one light. For recreational riding I use just the one on my helmet, but for racing flat out, I absolutely need both. And I have incredible night vision, so that’s not the issue, it’s just nice to have the trail lit up with the static light, and have a second light for where I’m looking.
Maybe I’ll notice a difference if I get faster. Right now, I simply cannot outride my light, even on downhill segments.
When you get you MS 1400, you should try a trail with your MS 900 turned off and let us know how it goes. I bet 1100 lumens on your bars will be enough, 99.9% of the time.