I saw the mystic. When Vin voltage is higer than 6.5V the lmv358 like shut down the atmega8u2 and serial wont work 
Maybe some know how to solve this.
PS.
In my project I use Motorshield with L293D chip which need higer voltage to drive motors 
It may be a stupid question but if i have a two seperate supply they never communicate through serial? If it - how can i communicate?