Effect of using buck converters in parallel
No real benefit is achieved. The buck that regulates to a slightly higher output voltage wins and the other buck basically switches off. However, it's probably a bit worse with synchronous buck regulators because they have active high and low mosfets and can act as a shunt regulator. 

This is likely to be a no win situation for either buck with one device trying to regulate downwards by shorting the output whilst the other will try and connect it's series pass transistor to the incoming supply rail far too often in order to fight the other device.

My advice is get chips that can work in master-slave mode or design a better buck regulator.