had simular problem with my 4870 and 7890 on one machine,never get manage to work em toghether,thing is legacy drivers for 48xx serie just wont work along new AMD drivers on one machine,I tried every posible solution but nothing helped all miner were crashing on start.
not sure what error message you getting
but there are some posible solutions now
https://bitcointalk.org/index.php?topic=175391.0also these steps helped for some
0) Download and run amd_cleanup_util_1.2.1.0.exe, use google to find it
1) Download Catalyst from amd.com
2) Run it
3) When it asks 'where you want.. blah-blah.. to extract?' specify "C:\temp\bzzz"
4) After unpacking finished and it asks you to press "next", press "cancel" instead
5) Press "Windows button"+PAUSE, open Hardware(win xp)->"Device Manager" or Control Panel->System->Device Manager
6) Find your GPU under 'Display adapters'
7) right click->update->browse my computer->let me pick from a list->have disk->C:\temp\bzzz\AMD\Support\
12-6_vista_win7_64_dd_ccc_whql\Packages\Drivers\Display\W76A_INF -> OK
8_) After installation go to Control Panel->Administrative tools->Services and disable 'AMD External Events service'
9*) If after all this cgminer/GPUZ reports that there is no OpenCL, go to C:\temp\bzzz\AMD\Support\12-6_vista_win7_64_dd_ccc_whql\Packages\Apps\OpenCL64 and run OpenCL.msi
path in step 7 could be different.