Gotcha. I did this and it worked:
subsettedList <- subsetSwitchAnalyzeRlist(
switchAnalyzeRlist = SwitchList,
subset = (ssSwitchList[["isoformFeatures"]]$condition_1 %in% c("NTC_noIR", "KD_noIR") &
ssSwitchList[["isoformFeatures"]]$condition_2 %in% c("NTC_IR", "KD_noIR", "KD_IR"))
)