best is to keep the limit price far enough from the trigger price so that the trade will execute at market price positively
In Nifty, I keep trigger only, and leave the other window as blank, meaning next available price. Sometimes takes 20 to 50 paise more, but execution is guarenteed.
Not advisable in stocks, as sometimes even liquid stocks cant handle volume. Have had a bad experience with aptech a few years back. Had an order for 2000 aptech at trigger 93. Sold at 91. From that day onwards, I have never traded stocks with an open market trigger.