// step one step:
myStepper.step(10);
If you are not going to keep the comments up to date, why leave them in?
Serial.print("steps:" );
Serial.println(stepCount);
stepCount++;
This doesn't match what you have actually stepped.
but i need them to work at the same time. how wood i do this?
You can't make two steppers step at the same time. Only one can step at a time.
If you are trying to build some kind of CNC machine, there are ways to microstep each motor so that the resulting motion of the pair approximates the motion you are trying to achieve. The actual motion will still involve stair-stepping, but that is true of any CNC machine. The key is to make the steps small enough.
There is nothing in your code that causes the other stepper (mySteppera is poor name) to ever move.