Synchronised Dimming and brightening of 2 x 12v LED strips using an UNO

create a simple little loop with a bit of math:

for(x=0 x<256; x=x+1){
brightness1 = x; // 0 to 255
brightness2 = 255-x; // 255 to 0
analogWrite(pwm1, brightness1);
analogWrite (pwm2, brightness2);
delay (352); // 90000 sec/255 steps
}