Uhhh...dont know about the omoplata but heel hook is a great counter to scissor sweep. To counter this counter you gotta connect to the guy as you're sweeping and keep him tight so he cant sit away for the heel hook.
If you're late and lazy you can also figure four leg lock both of their legs above their knees as they fall and get around to setting up a leg lock when you feel like it. It's a weird position though.