Multiple buck converters on the same PCB, should switching frequency be the same?
As far as I know this would be better due to the EMI and SI issues, it's easier to get rid off one frequency than multiple of them and their harmonics, products etc.


This is a false premise. EMI regulations limit the emissions on a frequency-by-frequency basis. If you have two sources in your system at the same frequency, their outputs can add, giving higher emissions at that frequency. If they're at different frequencies, they are effectively independent for purposes of emissions. 

It's a general rule in EMI design that the best solution is to reduce your sources of emissions, rather than generate signals and then try to block them. So I'd say for EMI purposes, you're better off using different frequencies for your different switching regulators.