[BillyMac]

Originally Posted by Nevadaref

3. If there is a simultaneous violation by each team, the ball becomes dead and no point can be scored. Remaining free throws are administered or play is resumed by the team entitled to the alternating-possession throw-in from the designated out-of-bounds spot nearest to where the *simultaneous violation occurred.
4. If there is a violation first by the free-thrower's opponent followed by the free thrower or a teammate:
a. If both offenders are in a marked lane-space, the second violation is ignored, as in penalty item (2).
b. If the second violation is by the free thrower or a teammate behind the free-throw line extended and/or the three-point line, both violations are penalized, as in penalty item (3).”
You had the situation described in 4b, so refer to part 3 for how to penalize and resume the game.

Originally Posted by crosscountry55

... presuming you don’t judge that the violation by B distracted free thrower A, thus causing him/her to violate.

9.1.3 SITUATION G: While A1 is attempting a final free throw, (a) B1 enters the lane too soon followed by A2, both of whom are in marked lane spaces; or (b) B1, in a marked lane space enters the lane too soon, then shooter A1 steps on the free-throw line while releasing the throw. RULING: In (a), the violation by A2 is ignored and, if the try is successful, the goal shall count and the violation by B1, shall be ignored. If the try is unsuccessful, the ball shall become dead when the free throw ends and a substitute free throw shall be attempted by A1 under the same conditions as those for the original free throw. In (b), a double violation is called and the ball is put in play using the alternating-possession procedure. COMMENT: Anytime the defense violates first, followed by a violation by the free-throw shooter, the officials should consider the possibility of disconcertion. (9-1 Penalty)