WordStar can only have either of two views of one document, or two documents at any one time. To implement this correctly Word would have to be limited in the same way (Word can have as many documents as you can fit in memory and any or all of them can have split window views).
Therefore File, Open is assigned to the standard Windows menu shortcut of ALT+F, O. Once the window is split, ^OK will move the cursor between the two editing windows just as in WordStar. ^OM will close the split (this is simplified from the WordStar options).