Sorry for a very basic question. How do you set the duty cycle in the code you provided?
The duty cycle is set by the OCR2A and OCR2B values. I'm unclear about the phasing requirements when the pulses are not 50% and each one is half the time period. You may need to use a dual sloped pwm mode. Using two different timers but synchronized might also be an option.
Now, how do I reduce the amplitude from 5V to 1.5V without any external hardware?
You will not be able to do that. Switching a 1.5 v source, like you are doing, or using an external voltage divider are your basic options.
What are you trying to achieve? What are you doing with the 1.5v square waves.