ATC rappelling

Is it ok to hook up an ATC like this to increase friction on a rappel

No. Use a hollow block or a prusic. 

Why not? There are several configurations of Z-rappels set up in a similar manner. Test it at your home crag.

That particular setup looks like it may have too much friction, but try it out.

Edit: It would prematurely wear out the ridge separating the two rope slots, so keep an eye on that.

Use two biners, which is what black diamond would recommend. They would probably not recommend whatever is going on there.

You will have more luck and ease by adding additional lockers for a higher friction. You are not going to die with your configuration, but I suspect you will be in pig tail hell on the next belay.

As an alternative to using two lockers (the preferred approach IMO), you could redirect the BRAKE strand (maybe through a biner on your leg loop) rather than the load strand as you are showing here (which looks like it has the potential to strip the sheath off the rope).

I prefer methods where you can add/change friction while rappeling.

https://www.mountainproject.com/forum/topic/117375875/stacked-atcs-for-lots-of-friction?page=2#ForumMessage-117380984

Adam Larkin wrote:

Is it ok to hook up an ATC like this to increase friction on a rappel

How often do you rap on a single strand?

I do a lot, nearly all the time in fact. Free-hanging (and being fat) on a single 9mm I just take a wrap around the biner.

This looks like a bad idea to me for two reasons. (1) Probably way too much friction unless the rope is very thin. (2) Excessive wear to the sheath from dragging a loaded rope over a relatively sharp edge.

Offhand, I can think of six different ways of increasing friction (not counting doubling up the device or any type of body wrap).  Some of these have to be instituted before the rappel starts, others can be deployed while on rappel. Any one of them is better than the proposed hack.

1. Double carabiners on rap device.
2. Brake strand redirected through carabiner on leg loop.
3. Z-rig: brake strand runs down to carabiner on leg loop, up to carabiner on rap device, down to brake hand.  Best to be using an extended rappel and have the lower carabiner on harness belay loop rather than leg loop.  This extends #2.
4. Extra twist(s) around spine of rap device carabiner.
5. Carabiner crossbar, Push brake strand bight through rap device carabiner, clip a second carabiner to bight and orient so spine bears the load.
6. Plaquette trick: you need to be using a guide-type device for this.  Clip the hole used for suspending the device when used in guide mode back to rap device attachment point.  This tilts the device back and adds friction.
   
   

I think everyone should know a method they can deploy while on rappel in case they discover after starting that they aren't getting enough friction.  Number 3 seems to me to be the best for this; Number 5 works too, as does Number 6, if you are using a guide-type plaquette, with a little effort.

Please post a video of your attempt. I would be amazed if you could move at all with that setup.

This is how I was taught to rig a super skinny line (Petzl Rad) for a single strand rappel on an AMGA course and have practiced with both this and a super munter.  I think the new super skinny lines are a special case and would add friction with one of the other methods already mentioned if I were using a typical size rope.

But there’s tiny ATCs now for skinny lines

What is going on with that bike in the bg 

You would probably get answers that were more helpful to you if you explained more about why you want to do this.