x1    10
x1    100
x2    1000
x2    10000
x3    989
x4    345
x10    8767736477736
xx    234
xy    387889999

x1    1010
x1    1100
x1    900
x1    800
x1    10000
x2    2000
x2    3000
x3    1989
x3    2000
x4    1345
x4    1500
x10    8767736478730
x10    10000000
xx    1234
xy    387888999

x1    10    x1    800    [x1  10+/-1000=1010 or -990 i.e all the values bw 1010 and -990 with a similar key x1 those are 800,900,1010 ]
x1    10    x1    900
x1    10    x1    1010    [x1  100+/-1000=1100 or -900 i.e all the values bw 1100 and -900 with a similar key x1 those are 800,900,1010,1100 ]
x1    100    x1    1100
x1    100    x1    800
x1    100    x1    900
x1    100    x1    1010
x2    1000    x2    2000       [x2  1000+/-1000=2000 or 0 i.e all the values bw 2000 and 0 with a similar key x2 those are 2000 ]
x2    10000    NULL    NULL                [Have none]
x3    989    x3    1989         [x3  989+/-1000=1989 or -11 i.e all the values bw 1989 and -11 with a similar key x3 those are 1989]
x4    345    x4    1345         [x4  345+/-1000=1345 or -665 i.e all the values bw 1345 and -665 with a similar key x4 those are 1345]
x10    8767736477736    x10    8767736478730                   [same like above]
xx    234    xx    1234                                                     [same like above]
xy    387889999    xy    387888999                                   [same like above]

awk 'NR==FNR{a[$1]=$2;next} a[$1] && ($2 <= a[$1]+1000) && ($2 >= a[$1]-1000) {print a[$1], $0}' test1.txt test2.txt

100  x1    1010
100  x1    1100
100  x1    900
100  x1    800
989  x3    1989
345  x4    1345
8767736477736  x10    8767736478730
234  xx    1234
387889999	xy    387888999