Unix: Replace line with string in another file

Could you please try following, written and tested with shown samples in GNU awk. This will handle multiple occurrence/values in file1.txt and fil2.txt too.

awk -F'=' '
BEGIN{ FS=OFS="=" }
FNR==NR{
  arr[++count]=$0
  next
}
/^ip/{
  print $1 OFS arr[++count1]
  next
}
1' file1.txt file2.txt

Explanation: Adding detailed explanation for above.

awk  '                      ##Starting awk program from here.
BEGIN{ FS=OFS="=" }
FNR==NR{                   ##Checking condition FNR==NR which will be TRUE when file1.txt is being read.
  arr[++count]=$0          ##Creating arr with index of increasing value of count and its value is current line.
  next                     ##next will skip all further statements from here.
}
/^ip/{                     ##Checking condition if line starts from ip then do following.
  print $1 OFS arr[++count1]   ##Printing current line and arr with index of increasing value of count1.
  next                     ##next will skip all further statements from here.
}
1                          ##1 will print all lines here.
' file1.txt file2.txt      ##Mentioning Input_file names here.