For Windows, there is a commercial application called BeyondCompare and an open source application called WinMerge. Both applications show two files side by side along with their line numbers. It's possible to select blocks of code from either window and move it to the opposite window and save the two files separately after completing changes. The general operation of the programs is simple and intuitive. I mention this only from the standpoint of using the general concept as a possible solution. However, a standalone application with the same functionality is also possible. For example, if Textastic supported optional ad-on modules, such a module could be offered as an upgrade.
I would *love* to see a side by side diff tool on my iPad pro. I need to do one right now and am not finding a good app to do that with. Have to compare production with proposed production from another developer.
Customer support service by UserEcho