An employer can appoint as many PRSA providers as they like, if they want to offer their employees a choice.
Each provider would be debiting the employers account each month with the amount that is due to each of them. This will have been collected via payroll from employees and employer pays their part(if any). This amount is confirmed to the payroll 'department' by each provider.
The only problems that you could encounter is that the employer or the payroll 'department' may deem it to be top heavy, administratively.