Process Control and SIS Integration

We are going to design a control and safety instrument system SIS (ESD / F&G) for an Oil treatment train composed from:-

1- The foreseen number of input/ output signals is less than 500.

My question is, do we have to design two separate control systems (one non SIL for process control and one SIL certified system for ESDF&G)

Or it is accepted from the international standards to design just one SIL system for both process and ESD/F&G, and if it is accepted , What is the constraints to be considered?

if we fully integrate the two system , do we have to use dedicating a CPU for the

process control and another the ESD/F&G functions.?

thanks