Moodle was not designed for the type of activity you describe. The only split-screen part of Moodle I can think of is the grading form for online text assignments. But that is not a good solution for your situation - there is no way to save the translated text externally.
If the purpose of the activity is to create document translations, then I suggest using a word processor in split screen mode. Or, you could post the original PDF document in Moodle and instruct the SMEs to open MS Word for their translation (resize the Moodle/MS Word windows so both can be seen).