How to implement side-by-side translation?
There hasn't been a whole lot of public work on this that I can find.

Some module options:


Side-by-Side Translation (no code)
Entity Translation Side-by-side Editor (code, no release)
Dual-Language Editor (code, no release) 


liquidcms helped me find that last one.

I'd recommend trying the latter two and seeing how far you get.  Post patches for missing functionality as necessary.

Also, see the issue Create Node With Multiple Translations Using Single Form.