I have Small form Optiplex 9010 with Radeon card (DIV & HDMI) and two display-ports on the motherboard.
Got two Dell U2312HM screens with display ports.
Start computer and go to Bios settings [F12]
In Bios settings:
Select Displays and choose for Radeon Graphics card (not on auto) as primary
Now reboot And go in the Bios again [F12]
Multiple display option is now available, so select it.
Now reboot again [F12] and enter the bios again.
Select Intel as primary
So the trick is to reboot after every single bios setting, since the new options will be availaiable after the reboot.
Now I suddenly got 5 screens on in Windows Display settings, but you can only get maximum 0f 3 active displays (e.g. 2 x display port and 1x DVI from Radeon card) or (2 x display port and 1 vga on motherboard.)
I think that the Additional Radeon card is also necessay even if you use all motherboard connectors (2 x display port + 1 vga setup), but did not test it.