I would have told you if I knew the truth. What is the rule here? ( second and thirs conditional together)
I have found examples like the one above. I know we shoud say either :
I would tell you if I knew or I would have told you if I had known.
But is it ok or common to use both conditionals together in one sentence?
What is the difference between doing so ( if it's possible) using each conditional alone?
Thank you so much :)

Question

I would have told you if I knew the truth. What is the rule here? ( second and thirs conditional together) 
I have found examples like the one above. I know we shoud say either :
I would tell you if I knew or I would have told you if I had known.
But is it ok or common to use both conditionals together in one sentence? 
What is the difference between doing so ( if it's possible) using each conditional alone?
Thank you so much :)

Answer

It is pretty common for the unreal conditionals to be mixed, but the meaning changes if you mix them.  

2nd: If I knew the truth, I would tell you (but I don't know the truth, so I can't tell you).

3rd: If I had known the truth, I would have told you (but I didn't know the truth, so I didn't tell you).

This case (mixed): If I knew the truth, I would have told you (but I don't know the truth, so I didn't tell you).

How you mix them depends on what what meaning you want to convey.  
There are more examples on this page: http://www.englishpage.com/conditional/mixedconditional.html

Hope that helps!

Answer

In this case the difference is irrelevant. Correct would be:
I would have told you if I had known.

However, if you knew something in the past, then you also know it in the present. This brings about the mix up when people talk. 
"I would have told you if I knew..." is simply a mix up in the speaker's mind.