S70FL01GS is a dual die device and the 2 chip select lines are to select each of them separately. If you implement CS#1=!CS#2, then one of the CS pins will always be LOW, so one of the dies will be active all the time. So, our device may consume more power (than standby state). If you are OK with the extra power consumption, then you can implement CS#1=!CS#2.
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