I was so busy at first trying to make sure the roaches didn't reach the same row or column as me that at first I didn't notice...
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×that you can let ONE roach reach the same row or column as you, and still make it to the tunnels.
Also...
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×You may have noticed that if both roaches reach the same row and column as you, you can still move diagonally around the screen for as long as you like, as long as you stay on the dark colored squares and keep moving. So, save trapdoors corresponding to those squares for last.