English-French & French-English
does anyone know if these courses can be used simultaneously?
Yep definitely. Not sure why you got downvoted for this, OP. It's a perfectly valid post!
For English to appear (on your profile.) you will need to be on that course or another 'from French course.' But when your commenting both will appear. :)
Here is the link to the official Duolingo Help Centre for
"How can I learn multiple languages at the same time?"
Yes you can do this. If you are on the English to French course, Just go to duolingo.com/courses and you will have to change your language. Choose I speak... (located on the top right of the page) and select French, then click on learn English course. After I finished the Spanish tree, i decided to change my language to Spanish and do the spanish to english course. It helped me so much with strengthening my reading and writing in Spanish. When you are ready to go back to your English learning French course you will have to go back to duolingo.com/courses and change the language back to English.