Step 1. Switch to the Spanish course. (Or switch to one of the other courses which has a "Words" page — German, French, Portuguese, or Italian.)
Step 2. Duplicate tab. (Right-click on your browser tab and select "duplicate" or "duplicate tab".)
You should now have two browser tabs open, both displaying the Spanish course "Home" page.
Step 3. Current tab: Click "Words".
Step 4. Other tab: Switch to the Japanese course. (Once it has changed to the Japanese course, you can close this tab if you like.)
Step 5. Back on the previous tab: Duplicate this tab.
Step 6. Enjoy! ^^