Eating Feces

Confine and supervise!  Ensure there is no way on gods clean earth he can go without your knowledge, when he goes in the right place give him a yummy stinky treat and while he chows down, clear the poo away.  If you can prevent it happening for long enough it is likely to break the habit. 
 
In the meantime, a vet check to rule out an underlying medical cause would be a good idea.  What is he eating?  Perhaps his body is not absorbing it efficiently, causing him to excrete it while still containing large amounts of valuable nutrients and still smelling like food to the dog.  Low quality diets are a common cause for this behaviour.... have you tried switching his food?
 
There are also things you can add to the diet to make the poo unpalatable.  I think pineapple is one.... there are also tablets you can get from the pet shop I think.
 
Obviously, never punish him for any mistakes in the house.... otherwise he will just be neurotic about toileting... if he has ever been scolded for it it might literally be that he is trying to "eat the evidence" because he is anxious about the presence of the poo.

Some dogs do that because boredoom, do they do any activities? do you take them out for walks?

That's the weird thing, we feed them Fromm (which is supposedly a premium dog food) and they get a lot of excercise with each other and us.  They also get a lot of attention because although we both work we come home at lunch so they are never left alone for more than 4-5 hours at a time. I think the problem did start because he was being scolded for pooing in the house, so I do take blame for that.  Just turning the problem around is frustrating!  I might try the pineapple (I have seen that on another website as well) but I don't like adding too much to their food.  I will also try taking him out on the leash for the next few days.

Try this link.

[linkhttp://www.dogplay.com/Behavior/poop.html]http://www.dogplay.com/Behavior/poop.html[/link]
 
The 1st two links don't work as well as the 2nd from last.

"Coprophagia" is the term for stool eating.  Put that word into a search (same line as the login) and you will get a number of threads.
 
I believe that the best ways to solve the problem are
(1)  breaking the habit via close supervision and
(2)  finding a food that the dogs can digest very well (adding digestive aids to help with non-meat food items may help)

ORIGINAL: janet_rose
(2)  finding a food that the dogs can digest very well (adding digestive aids to help with non-meat food items may help)

 
How do you tell if the food is not digesting well?

ORIGINAL: Chuffy
Low quality diets are a common cause for this behaviour.... have you tried switching his food?

[&:] I don't know Chuffy.... One of my Tzus has done this on and off since he was a puppy.  They have always been on a premium food.  First Wellness Puppy Mix, next  EVO, now NB Duck/Potato.  He doesn't eat his own anymore, just his brother's sometimes (so maybe feed the Pineapple to his brother?!)

It's difficult to stay on top of it because he'll go for a long stretches of time without doing it, and then he just goes through a period where it's the best stuff on earth! [:'(] 

We have an acre fenced in that they run around and play in.  I guess the only way to be close enough to scoop it up is to take them out on leash until I see them go and then clean it up...otherwise, they run way out and then I can't "catch him" before he digs in!

They're Nuts...  I just do my best to supervise and intervene when I see it happen.  He was actually throwing up brown liquid when I realized he had picked up the old habit again.

It's not the only cause.... just a common one. 
 
Out of the five dogs I know who are on a raw diet (not mine) three of them used to be poo-eaters.  This changed with the switch to raw.  Without exception, they all excrete much less now - down by roughly almost a half.  It's kind of drier and there is much less odour too.  Perhaps these are signs of a food that the dog is digesting well and efficiently?

How do you tell if the food is not digesting well?

They don't have small, firm poops.

ORIGINAL: Chuffy

It's not the only cause.... just a common one. 

Out of the five dogs I know who are on a raw diet (not mine) three of them used to be poo-eaters.  This changed with the switch to raw.  Without exception, they all excrete much less now - down by roughly almost a half.  It's kind of drier and there is much less odour too.  Perhaps these are signs of a food that the dog is digesting well and efficiently?

Ok we have fed both dogs some pineapple chunks mixed with their food (Fromm dry) and we have caught both of them going after the poo.  It must be a psychological reason why they're doing it... the younger one will go after it outside AND inside, but he is pooing more often inside the house lately and wanting to eat it.  I HAVE noticed, however, that their poos have been a little bit smaller, maybe due to the pineapple? Not sure.  The size really doesn't matter to me if they're gonna chow down on it []