I got frustrated with this, but after running it around 10-12 times, I finally got it to work. I tried all the methods listed, but the one that worked was the charge right out of the gate. It took a couple tries to hit both in the initial charge, but if you do, they will agro. Try waiting until they are walking towards one another, have the f key already pressed and hover your mouse on the menu option to confirm opening the gate. As soon as you click it, begin charging. If you hit both, you will be able to move onto the next wave of enemies.