Steering wheel with optical or multiturn

"Do you know a dedicated IC that has only the count (in both direction) features"?

You may be surprised to learn that a few simple, cheap, generic logic chips are sufficient to decode a rotary encoder -- it doesn't require some special chip specifically customized for rotary decoders. The Codewheel Generator Page has a few relatively simple circuits that decode the 2 rotary quadrature outputs into a "direction" and "count" outputs. Those outputs can be wired into the "direction" and "clock" inputs of any up/down counter -- such as, for example, the CD4029, the CD4516, the CD40193, 74HC193, etc.

"is it possible to make à simple one with an attiny"?

Yes, you can make a simple quadrature encoder with an ATtiny. Some tips for connecting a quadrature encoder to a Atmel chip and decoding it in software are at:

• Alex Brown's Robotics Page: Using Encoders
• Arduino Playground wiki: Reading Rotary Encoders
• Keith's Electronics Blog: LED Calculator with Rotary Quadrature Encoder

If you have relatively few pulses-per-second, you might be able to run the 2 signals from each encoder directly into your main ATmega and use the above techniques to do it all in software, without any external conversion hardware.

Your while loop logic is incompatible with the conditions inside it.

Your while loop is saying:

as long as BOTH wheel encoders read less than clicks, execute the code inside

and your if statements are saying:

if any of the wheel encoders is greater or equal to clicks, stop the corresponding motor

This is generally false except for the rare condition where one of the encoders happen to increment between the arduino evaluating the while and the if statements. In order words just about never.

So the first thing you need to do is fix your logic. Then you need to investigate more sophisticated means of keeping the wheels in sync, such as computing an error term from the different in ticks, and employing a PID controller algorithm to adjust the speed of both motors via analogWrite.