Teams decide which side of the defense plays Cover 4 and which side plays Cover 2 based on tendencies and what they think best defends against what they feel the offense will do.
Scenario 1) In this scenario, we are leaving Von Miller (E for edge rusher) out of coverage so he can chase and harass QB’s, The left hand side (facing) is running the Cover 4, and the right hand side is running the Cover 2. The RCB has a short curl/flat zone and the Sam, Mike & Will have the other 3 under zones.
Scenario 2) Again, Miller is running loose, but the responsibilities have flipped with the left hand side is running Cover 2 and the right hand side is running Cover 4. Everything is flipped from a secondary concept.
Scenario 3) Once again, #58 is chasing the QB but in this variation the LCB is playing man to man coverage on the X receiver. If it’s Aqib Talib, maybe he will leave Michael Crabtree’s chain alone. The Mike (Brandon Marshall) has a larger Middle/Hook zone with the the two outside under zones being covered by the Sam & Will backer. The RCB & SS have the deep 1/4 while the FS is manning a deep 1/2, on the left side of the field.
Scenario 4) In this scenario, Joe Woods is leaving the job of applying pressure to the QB to the front 3. The two other changes in this scenario from scenario 3 is that Miller (E) is responsible for a shot curl/flat zone, and that Middle Hook is divided between the WIll and the Mike.
There are scenarios where the under zones could be handled differently with pressure coming from a linebacker blitz, or even a corner blitz, but blitzing will be addressed in another post.
This is an example of how a DC can take parts of two separate defensive schemes and make an entirely different one. It would be compared to an OC taking an I formation running play and moving the FB out to wing/TE position and knocking the QB back to the shotgun but having the same passing routes out of the 5 eligible receivers. As long as the defenders know what their keys (reads) and responsibilities/assignments are, then they can mix and match various concepts and assign that new defensive scheme/play a new name to keep it different.
So in the end, no it’s not Cover 2 + Cover 4 = Cover 6, but it’s more Half Ham & Cheese combined with half Turkey & Cheese with onions.