Well, the Radiant could sleep while their spren kept watch.
We don't know that they can't form into armor. The spren travelling with the Radiant is no more ridiculous in the Cognitive than it is in the Physical.
Well, A-aluminum doesn't work like that. It gets rid of the alien investiture in the soul. Even if the aluminum twin managed to hurt the Radiant's Shardblade, they would still instantly die.
Granted. Here is chocolate. Your Bane is that your older brother will constantly question whether you like chocolate or not.
I wish for the ability to gain any knowledge, (age, pregnancy, etc.) that is needed to keep me from commiting any huge social missteps.
So, normally, when you have a conjoined fabrial, the two pieces will move in exactly the same way. When you touch the fabrial with aluminum, you can make the two fabrials ignore certain types of movements.