... why would I need a second yn622 for the speed lights if they are already triggered by the yn622?
The YN685 has a built in radio receiver for the signal broadcast by the YN622-TX. The YN560 IV does not.
The radio in the YN560 IV is on the YN560/RF605/RF603 II/RF603/RF602 system which is not compatible with the YN622 system. And while Yongnuo has added YN560/RF605/RF603/RF602 receiver capability to YN622 triggers and flashes with built in receivers made since mid-December 2014, it has not added YN622 receiver capability to the receivers in YN560/RF605/RF603/RF602 receivers and flashes.
The YN560/RF605/RF603/RF602 system is for manual only control. No TTL, 2nd curtain sync, HSS, etc. The YN560 also allows remote setting of manual power and zoom head settings and multiple groups. The RF605 includes the group functionality. Otherwise, the YN560/RF605/RF603/RF602 system only fires the flashes via radio. When YN622 triggers and flashes have their radios set to YN560/RF605/RF603/RF602 compatibility mode, they are also constrained by the limits of the YN560/RF605/RF603/RF602 system.
The YN622 system is for full flash functionality. When a flash is connected to the camera via the YN622 radio system, it is just as if the flash is sitting on top of the hot shoe with regard to TTL, HSS, 2nd curtain sync, etc. This protocol is obviously more complex than the manual only system.
If you don't need TTL, HSS, 2nd curtain sync, etc. then you might consider using a YN560-TX controller. The YN685 and YN560 IV speedlights will not need a receiver as all can be set and fired manually by the YN560-TX. If your camera has a PC terminal then you can trigger the older CST via it (The CST2 has no PC sync port). Or you can use an RF603 II or RF605 connected to your camera's hot shoe to trigger the Yongnuo flashes and place a CST2 on top of the RF603 II or RF605. The RF603 II and RF605 have a pass through (manual 'fire" only) hot shoe that should allow you to stack the CST2 on top of the RF603 II or RF605 mounted on the camera's hot shoe, but I've not confirmed that anyone has been successful doing that specific combination.