This advance trigger would alert you when the combined premium of both the strikes goes over 1000 -
(LastTradedPrice('NFO:BANKNIFTY21JUN34600CE') + LastTradedPrice('NFO:BANKNIFTY21JUN34600PE')) >= 1000
If your intention is to only get alerted when the combined premium of both the strikes goes below 1000, then try this -
(LastTradedPrice('NFO:BANKNIFTY21JUN34600CE') + LastTradedPrice('NFO:BANKNIFTY21JUN34600PE')) < 1000
Also check this thread for reference.