I know I can use Awk, but I am on a Windows box, and I am making a function for others that may not have Awk. I also know I can write a C program, but I would love not to have s
If scripts don't do it for you, you could record the actions to a register (the carriage returns are added for readability):
qa
1G0
xGop
1G0j
xGp
q
This will give you a macro that you could run against the example above, or any 2-line strings of the same length. You only need to know the length of the string so you can iterate the operation the correct number of time
16@a
A fairly basic solution, but it works.