You use both at the same time. One actually is different than the other. I hardly ever use full strength nutes at any time so half it for a bit and build up from there. Big Bloom is just liquid earthworm castings and bat guano and are easily digested by good roots so burning the plants is tough to do with it. If you add beneficial bacteria to the mix, you'll have more success with them. You might want to add a little Grow Big for Nitrogen, like 25-50% of the normal dose. FF doesn't put much Nitrogen in their bloom nutes.