I have (or had) the same issue as you. I have an HD6950 2GB for gaming and a FirePro3D V5700 for CAD. This worked well until last fall when AMD stopped supporting the HD2xxx, HD3xxx, HD4xxx and all FirePro3D cards older than the V5800.
My temporary work around that worked a couple of months until Catalyst 12.10 was to hack the inf for the gaming version of my CAD card, which is the HD4650 (RV730), and install the core driver for 12.6 for the V5700 (using the driver for the HD4650 with the INF modified to make the V5700 be detected as an HD4650). To my surprise this not only allowed both cards to work side by side, but the HD4650 driver allowed the advanced CAD features to work, enabling enhanced graphics features with Solid Works. The Catalyst Control Center even showed the V5700's advanced Open GL settings that could be controlled that are only available on FirePro3D cards.
Unfortunately when Catalyst 12.10 came out the new Catalyst Control Center was no longer able to control the old 12.6 driver used with my V5700. Whenever CCC tried to control the V5700 the system would lock up.
I searched for a solution for a long time and eventually came up with dual booting two copies of the OS (on different partitions). One boot had the actual V5700 driver installed and the HD6950 disabled, while the other had the opposite. Unfortunately for some reason with the V5700 disabled and 12.10 running the HD6950 the system remained unstable so eventually I pulled the V5700 and just use the HD6950 with reduced functionality for CAD. It turns out that the system is still having graphics locking up issues so my HD6950 might be failing (but that is another story...).
For your system are you not just as well off to uninstall the HD3850 and use the HD5870 for both monitors? I would have thought the HD3850 would have bottlenecked the system (if it was actually being used for graphics processing) and actually produced slower frame rates than just using the HD5870. Most of the time the AMD driver only uses a single video card for all of the image processing and the second card is only used for displaying what the other generates anyway.
What I discovered when I was using both cards is that if you use the Catalyst Control Center to configure the displays that the primary display card became also the graphics engine for both cards. The other card becomes just a display card without any graphics processing. If instead of using Catalyst to control the cards you used the Windows 7 display management software in the control panel without CCC installed, it could actually be configured to use both graphics chips - but in my case this produced much slower results than using the HD6950's chip for both displays.
09:53 AM by