One light on the bars and one light on the helmet is the way to go.
With lower power lights, I agree.
With a well-designed 900 lumen light on the bars though, you’d have to be very fast to outride your light. Downhill racing is the only case I can think of where you’d need more than that. I only want a helmet light for convenience: looking off the trail, mechanical issues, getting stuff out of my pack, etc.
I will be running my 250 at low power (110 lumens?) unless my 900 dies.