I use both switched on, from day one until harvest.
This works just fine, I believe better than just using the veg light on its own.
Technically, you use the the veg light for veg (duh!) and then the veg AND bloom light during flowering.
The bloom light is a supplement to the veg light and the bloom light is not designed to be used on its own as the only source of light during flowering.
Hope this helps, Organoman.