The SequencingADC will scan all channels (in your case only one) at soc except the "Injection Channel". This conversion you may start using the ADC_EnableInjection() API which will additionally scan it at the next soc. All can be handled quite easy within the eoc interrupt handler.
Bob, Please look at the graphic I attached. I have 2 channels + the Injection Channel although I confess I don't know what the Injection Channel is for. If I start a conversion during my 250 every second ISR (I think you're saying both will be converted), how will I get the 2 different results back?
Please read the first page of datasheet for "Injection Channel"
An optional “Injection channel” may also be enabled by firmware to occasionally scan a signal that does not need to be scanned at the same rate as other channels.
So your regular conversion channel is the mic and occasionally the Vbat.
To get the values, see list of APIs in DS.