The LBR does not speak to a legally occupied base. It only talks about "a base".
So, while both runners are not entitled to remain on the same base, neither runner will be in violation of the LBR so long as they both remain there. But, if F1 has the ball in the circle, and the ball is live, and F1 is not making a play, then either runner would be out if either left the base.
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Tom
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