Finally, I went with this:
public static List<string> combine_list_elements(List<string> lst_original, int i, int j)
{
if (lst_original.Count > i && lst_original.Count > j)
{
lst_original[i] += " " + lst_original[j];
lst_original.RemoveAt(j);
}
else
{
throw new Exception("One of these 2 items does not exist in this list.");
}
return lst_original;
}
It's simple enough, not sure why I was looking for else.