I don't trade with IB either, but once a price level is hit and an order is filled using OCA(OCO), the other orders are canceled. You should be able to break up the orders in 3 groups.
Sell 600 @ 30.50 Limit - Sell 600 @ 29.50 Stop OCA
Sell 400 @ 30.75 Limit - Sell 400 @ 29.50 Stop OCA...