How do you power the two Arduinos and the LCD display?
If via USB cable it might draw too much current (LCD backlight is very hungry) and then the capacitors which stored a bit of power are empty and the USB interface doesn't deliver enough to care for everything.
So, either use another USB port with more power or, the best solution: attach an external power supply (e.g. 7.5 .. 9V wall wart which can deliver 1000mA or more).
Thank you my friend for your help.. I connent each arduino with its own cable, only the screen connected to the uno and when I connect the uno and the mega (each its own cable) the screen work 2-3 seconds and stuck...but if I connect only the uno without the mega its work very good...You think an external power To UNO will help?