You just need to play the second string when needed.
I find that with certain students, especially older ones, that they are thinking too much. As that contradicts another thing I say, which is you have to think what you are doing and focus, I should try and sort this out!
This posting on Norman Lebrecht's site is very helpful. A German cellist lost most of his memory to a brain infection. He could no longer remember his own biographical details, his family, names of German rivers and so on. Severe memory loss. But, the fascinating thing is that he retained memory of how to play music. He could still play and sight read music on cello as well as he did before! To demonstrate this, ten amateur and professional cellists were used for comparison. The study concludes that:
I've been saying this in my teaching for years. Students who come to me unused to using the "brain networks" specific to music, need to learn to access them, to go inside the music part of the brain, or, as I more usually say, "You have to feel the flow and ride along with it..."These findings suggest that learning and retention of musical information depends on brain networks distinct from those involved in other types of episodic and semantic memory.
Back when I used to practice six hours a day, I noticed that after being deeply immersed for hours in music, when you left your practice room and returned to the real world, it seemed rather odd. Like landing on an alien planet! Music exists in another world. The most miraculous thing is that it is one we can all share.