R730xd how to connect rear flex hdd backplane without using more hba when front backplane in splitmode not unified?

Hi,

I am testing a R730xd with a hba330 minimono embedded controller and a hba330 pcie controller. The minimono is connected normally to one side of A/B connectors of front backplane. The hba330 pcie controller is connected to the opposite side of the front backplane to both the other A/B connectors where normally one of the two connectors would be used by the rear flex backplane.

After connecting both hba330 there are no more unused connectors on the front backplane to which I can connect and extend to the rear flex backplane. There are also no unused connectors on any of the hba330.

I would prefer not having to occupy another pcie slot with another hba or raid controller to be used with the two bay rear flex backplane. 

I tried to connect the rear flex backplane to the motherboard embedded sata ahci raid controller and this does not work with the rear flex backplane regardless how I configure the sata controller settings in bios.

Is there any method of correctly connecting the rear flex backplane to the onboard sata controller?

Thank you