Is it okay to use 2 teeth whitening kits in a row?

I recently used the Crest white strips Premium 7day kit and although it did whiten my teeth, I still want them to be whiter. So after 3 days I got another 7day kit and put them on but after a few minutes I got an insanely shock of pain in my teeth and I had to rip the strips off. Was this not safe to do? Did I just waste $40 or can I use them again down the road
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I would wait and not bleech them that much, they can get very sensitive!
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