Tedium in Column Highlighting

I had two files. One had records of type A, and one had records of type B. The only way to match them up was by line number. Row 40812 in the first file corresponds to row 40812 in the second. How to get them into one place so that I can import them into a table?

Excel is out -- I can't have it dropping leading zeros or messing with data in other ways, and I don't trust it not to monkey with something or other.

One of the things that I tried was to indent all of the values in the first file, then highlight several thousand at a time. Then, a tedious column highlight (alt + mouse on Windows, option + click on Mac. On Mac, you can let up on the option key after the mouse click, which helps the sudden-onset arthritis quite a bit.)

This was working for me, but long before I finished, the guy who generated the files created a single file in the format that I was trying to produce.