Hi I have komplete audio6. Can I use two headphone with it?
 
there only one headphone output. when I plug tow head phone the signal goes low!
 
This is my headphone
Audio technica ath m50x

Assuming you mean two headphones, where are you plugging the 2nd one in for the signal to drop?

I don't have the interface but it sounds like the single headphone output will not have enough power to run the two sets of headphones.  So if you need run two headphones you would need to buy a headphone amp that would be able to power the additional set of headphones.  With the Komplete Audio 6 having multiple outputs this would also mean you could set up alternate headphone mixes.

can you give me name of a good headphone amp? not so expensive i want something all use it

The komplete audio6 only has one phones socket.
Where are you plugging the second headphone in?
 
Cheap and good...
 
I have several Behringer HA400, they do the job.
 

Agree on the Behringer HA400
$25 https://www.amazon.com/Behringer-HA400-BEHRINGER-MICROAMP/dp/B000KIPT30/ref=sr_1_1?ie=UTF8&qid=1509790926&sr=8-1&keywords=behringer+ha400

Every time you passively split a signal, it drops by 3dB on both sides if the split.

I solved the same problem with a Behringer Mini Amp 800:
 
http://www.fullcompass.com/prod/080966-Behringer-MINIAMP-AMP800?utm_source=bing&utm_medium=cpc&utm_campaign=Product,%20all%20low%20bid(BSC)&utm_term=1100201936817&utm_content=Ad%20group%20All%20Products
 
I have been using this unit for years now with no problems.

gswitz
Every time you passively split a signal, it drops by 3dB on both sides if the split.

And that's the best-case scenario. Some audio interfaces don't provide enough current to drive even one low-impedance headphone (your ATH-M50s are 38 ohms). 
 
Almost any cheap headphone amplifier will mitigate your immediate volume problem. However, you might want to consider a slightly beefier amplifier for better fidelity. (No need to go out and get some expensive audiophile amp, though. Then you'd have to buy better headphones too.)