Correct way to initiate multiple ADC reads? (My second reading is causing a fault)

[escher]

I'm reading ADC1 and ADC2 in Dual Regular Simultaneous mode using DMA on a F334R8 nucleo board, as per instructions in UM1786 section 6.2.2 "Execution of ADC conversions":

void readADCs(void) {
  // enable ADC slave
  if (HAL_ADC_Start(&hadc2) != HAL_OK) {
    _print("can't start slave ADC\n");
  }
  // enable ADC master
  if (HAL_ADCEx_MultiModeStart_DMA(
	&hadc1,
	(uint32_t *)aADCDualConvertedValues,
	ADCCONVERTEDVALUES_BUFFER_SIZE
    	) != HAL_OK) {
    _print("can't start master ADC\n");
  }
}

The first reading works fine; I've checked the values and they're good. After reading finishes, I stop the ADC:

void HAL_ADC_ConvCpltCallback(ADC_HandleTypeDef *hadc) {
  if (HAL_ADCEx_MultiModeStop_DMA(&hadc1) != HAL_OK) {
    _print("can't stop ADC\n");
  }
  if (HAL_ADC_Stop_IT(&hadc2) != HAL_OK) {
    _print("can't stop slave ADC\n");
  }
}

I've checked with a print statement to verify the callback completes successfully. However, next call (after 2s delay) to readADCs fails (PRECISERR) on the first call to HAL_ADC_Start.

Here's the line that falis from stm32f3xx_hal_adc_ex.c:

HAL_StatusTypeDef HAL_ADC_Start(ADC_HandleTypeDef* hadc) {
  HAL_StatusTypeDef tmp_hal_status = HAL_OK;
 
   /* Check the parameters */
   assert_param(IS_ADC_ALL_INSTANCE(hadc->Instance));
 
   /* Perform ADC enable and conversion start if no conversion is on going */
  if (ADC_IS_CONVERSION_ONGOING_REGULAR(hadc) == RESET)
  // ABOVE LINE FAILS WITH FAULT

Clearly I'm not stopping and starting the ADCs in the correct way (despite having read some stmcubef3 examples). What am I doing wrong?

Bonus question: I only have to auto-calibrate the ADCs once per power cycle, right? (i.e. not once per read cycle?)

EDIT: I've realised that I only get the fault inside a freeRTOS thread, not when calling readADCs from main. I don't know why this would be.