Your YN-622C-TX will control both flashes directly via radio so neither flash will control the other in a master/slave way which is normally an optical system.
The YN-685 has a built in radio receiver that can be controlled from the YN-622C-TX without a YN-622C attached to the flash. The flash should be set to "622 Remote" mode by holding down the mode button until it shows "622 Remote" on the flash's LCD panel. The "Slave" symbol will also be displayed just to the right. Be sure to set the channel on the flash (choose from channels 1 through 7) to the same channel number displayed on the YN-622C-TX and to set the group (choose from groups A-B-C) to the group you wish to use on the YN-622C-TX to control the YN-685.
As an aside, I've tried without complete success to control the YN-685 with a YN-622C receiver and the flash set with the radio mode off. The changes I make to FEC in E-TTL mode on the YN-622C-TX attached to the camera look good on the YN-622C-TX control panel, but the flash ignores them and does whatever is set on the flash when it fires. (Even if they're at the needed defaults to allow wireless control, just as when in "622 remote" mode without the YN-622C.)
Note: I just tried experimenting again with the YN-685 radio turned off and attached to a YN-622C. Most functions work as they should when using the controls on the YN-622C-TX including changes made to power in manual power mode, but changes made to FEC in E-TTL mode do not change the setting on the flash: it stubbornly stays set to "0". FEC in E-TTL mode works as it should when using the YN-685 in "622 Remote" mode without the YN-622C attached.
You will continue to use a YN-622C attached to the Neewer TT680. The channel on the YN-622C should also be the same as the one set on the YN-622C-TX. If you wish to control each flash independently then you need to use one of the two remaining group settings to control it from the YN-622-TX. If your YN-685 is set to group "A", then you could use group "B" or group "C" to control the TT680 via the YN-622C attached to it. Just set the YN-622C attached to the TT680 to the group you desire.
A few other things you might not have discovered using the single TT680:
- If you want to control the flash from the YN622C-TX or directly from the flash control menu of your 6D then the menu on the flash needs to be preset to "neutral" settings. That is, zoom should be set to "Auto", flash power should be set to 1/1, flash exposure compensation should be set to "0" (zero), etc. If any of these settings are set to any other value via the flash's control panel the flash will hold that value no matter what you do via the flash menu on the 6D or the menu on the YN-622-TX.
Say you set the flash zoom to "24mm" using the control panel on the flash, but the other settings are all at the neutral default settings (1/1 power, 0 FEC). You will be able to control power and FEC vis the YN-622C-TX, but you won't be able to change the zoom setting from "24mm".
- Any changes you make using the menu of the YN-622C-TX will not be reflected on the LCD panel on the back of the flashes until you have half pressed the shutter button following the change to the settings. When using the flash control menu of the 6D pressing the "Set" button will do the same thing.
- You can control one group using manual power control and another group using E-TTL control.
- Note that flash head zoom can only be set using the YN-622C-TX at the same setting for all flashes: either as "Auto" that is set by the focal length reported by the camera or the same manually selected focal length. If you want each flash's zoom set to different values then set each flash independently via the flash's control panel. Of course if you have a flash's wide angle difusion panel pulled out or the head is set to any "bounce" angle the zoom function is disabled for that flash.
- If your camera only allows one Flash exposure Compensation (E-TTL) or Flash Power (Manual) setting via the external flash control menu, whatever setting you make there will only be applied to group "A" on the YN-622C-TX. Even if your camera allows for separate group A:B:C control of FEC or power, if you select "2nd curtain sync" using the YN-622C-TX, the camera's control menu will revert to a single setting which will only apply to group "A". (With the YN-622C-TX attached the "2nd curtain" option is greyed out on the camera's menu, but if you set it using the YN-622C-TX it will then show as selected on the camera's control panel!) To control groups "B" and "C" with 2nd curtain sync selected, or with an older camera with only one group in the camera's external flash control menu, you'll need to use the control panel on the YN-622C-TX.