Something like this?
Code:
$ cat test-case.txt
1 2 a b
1 2 c d
1 2 e f
1 3 a b
1 3 c d
$ cat gosha-awk
#!/usr/bin/awk -f
BEGIN {
col_1_prev = ""
col_2_prev = ""
}
{
if ( $1 == col_1_prev && $2 == col_2_prev )
printf "\t\t%s\t%s\n", $3, $4
else
printf "%s\t%s\t%s\t%s\n", $1, $2, $3, $4
col_1_prev = $1
col_2_prev = $2
}
$ ./gosha-awk test-case.txt
1 2 a b
c d
e f
1 3 a b
c d