With the hint here: https://github.com/in2code-de/powermail/issues/1197

This works:

temp.emailA = CASE
temp.emailA {
    key.data = GP:tx_powermail_pi1|field|fieldA
    12345678 = TEXT
    12345678.value = [email protected]
    default = TEXT
    default.value = [email protected]
}

temp.emailB = CASE
temp.emailB {
    key.data = GP:tx_powermail_pi1|field|fieldA
    key.stdWrap.crop = 3 | | 0
    123 = TEXT
    123.value = [email protected]
    default = TEXT
    default.value = [email protected]
}

plugin.tx_powermail.settings.setup.receiver.predefinedReceiver.receivers1.email = COA
plugin.tx_powermail.settings.setup.receiver.predefinedReceiver.receivers1.email {
  10 < temp.emailA
  20 = TEXT
  20.value = ,
  30 < temp.emailB
}

Thanks to https://stackoverflow.com/users/6783328/julian-hofmann