If I'm running a White Industries Eccentric Rear Hub with a freewheel on each side, what's the largest difference the two freewheels can have from each other? Could I run a 16 and a 20 or is that too wide of a range?

it kinda depends on the length of your dropouts/fork ends. 4 teeth difference might be getting dangerously close to the edge.

No, it doesn't. :|

You are correct, but only because you didn't make it apparently clear that your running it on a Verticle Dropout frame... Does the Eno even have threading for a freewheel on both sides? I haven't had the opportunity to run an Eno yet but I am on the hunt for one. I'd think that you should be able to get away with a 4 tooth difference, but there is no knowing with certainty until you try it out.

How much space does the Eno hub give you in a circumference?

White says you can order a Flip/flop Eno...I would say that the issue is in Chain length not anything else and 4 teeth is probably too much.

The only reason to run an eccentric rear hub is to apply chain tension on a frame with vertical dropouts.

In any case, I ordered two white industries freewheels that are 3 teeth apart. We'll see how well it works when they arrive.

It doesn't just depend on the teeth count difference, it also depends where your chain falls with each freewheel.

For example, I had a frame with short horizontal dropouts geared 42/15. Although the dropouts were long enough to accommodate a 6 tooth difference, there was no way I could run 42/15 and 42/16 on my flip flop because the chain would be too short in one or too long in another if I added a link.

Each tooth moves the axle 1/8". You can try measuring the hub in each position.

the erics eno hub is a fixed/free. you cant run freewheels on both sides... if you were doing a cog and freewheel 4 tooth might be too much for the hub to rotate to take up slack.

Freewheels can be run on both sides of a fixed/fixed hub, you just don't put lockrings on.