There is non-volatile memory (NVM) in each channel of the FADEC which stores various data including which channel was in control and which igniter was used on the previous start.
It's a simple matter for the FADEC to check NVM when it initializes and decide what to do for this start.
On the early generation FADECs, NVM was very limited (in fact overall memory was limited as well - the original PW2000 only had 24k of RAM memory per channel
). But the current generation of FADECs have lots of RAM and NVM)