Not a fan of Poliquin personally but i do this anyway. Coach doesn't want me doing Sheiko at the moment until we are sure the back issue is taken care of and the area strengthened. Back to training 4x a week: 3 days of bench or supplemental/accessory work, 2days of squat work, and 2 days of deadlift work....and there's a possibility I may add a 5th GPP day. That means I'm basically working every muscle group 4x a week and targeting weaknesses on the GPP day.
Nowhere in nature do you work a muscle and not it's antagonist. Try to think of an example without using machines at the gym. They learn to get stronger and bigger together not separately. Your quads are lagging behind your hams? Simple, don't add in 67 sets of leg extensions but add in front squats, switch from low bar to high bad squats, and sub in sumo deads for a macrocycle. Hams lagging behind quads? Simple, don't add 48sets of leg curls but go back to low bar squats, stick to mostly conventional deads, add box squats or paused squats or even pin squats, RDLs, SLDLs, snatch grip DL, etc.