This work is now (finally) complete!
If you roster Ohtani and visit your post-draft lineup page, you will see Shohei Ohtani in both the hitters section and the pitchers section.
Clicking on the Pitcher version will allow you to move him in and out of your Pitcher section of your lineup, and clicking on the Hitter version allows you to move him in and out of the Hitter section of your lineup.
In order to get hitting and pitching stats from Ohtani on Opening Day, he must be in a SP slot and the Util slot. If you only want hitting stats, he must be in the Util spot and on the pitching bench. If you only want his pitching stats, he must be in a SP slot and on the hitter bench.
Let me know if you have any questions!