vertical-merge.sh

#!/bin/bash

awk '
    FNR==NR {
      l = length($0);
      if (l > ml) {
        ml = l
      }
    }
    FNR!=NR {
      while (length(a[FNR]) < ml) {
        a[FNR] = a[FNR] " "
      }
    }
    {
      a[FNR] = a[FNR] $0
    }
    END {
      for (i = 1; i <= length(a); i++) {
        print a[i]
      }
    }
  ' \
  $1 $2