Code:
grep -iRI "string1" *.txt | grep -L "string2" *.txt
x:/var/tmp/test# cat 1.txt
lkjdfg
kljdfg
string1
jkhsd
x:/var/tmp/test# cat 2.txt
lkjdfg
kljdfg
string1
string2
jkhsd
x:/var/tmp/test# cat 3.txt
lkjdfg
kljdfg
string1
jkhsd
x:/var/tmp/test# grep -iRI "string1" *.txt | grep -L "string2" *.txt
1.txt
3.txt
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