I'm not sure if this is helpful or not, but you said "any" help. I'm using v2.11.2, in the window I push G and D, and can effectively disable Thread 0(GPU0) it seems. It may actually disable both cards, so I would suggest reading what it says when you hit G, and see if you can manually disable one of the GPUs.
You can also try making it a config file:
(You said the 5870 is overclocked, so change and check these settings to your proper setup and cgminer location for header files.)
{
"pools" : [
{
"url" : "http://stratum.give-me-ltc.com:3333",
"user" : "example",
"pass" : "xxx"
}
],
"intensity" : "d,1,18",
"gpu-engine" : "1250,1150",
"gpu-fan" : "0-85,0-85",
"gpu-memclock" : "875,700",
"gpu-powertune" : "10,0",
"temp-cutoff" : "95,95",
"temp-overheat" : "85,85",
"temp-target" : "75,75",
"auto-fan" : true,
"auto-gpu" : true,
"expiry" : "120",
"failover-only" : true,
"gpu-threads" : "1",
"log" : "5",
"queue" : "1",
"scan-time" : "60",
"temp-hysteresis" : "3",
"kernel-path" : "C:\Users\L\Desktop\cgminer-2.11.2-win32\"
}
Options for command line only:
--config|-c Load a JSON-format configuration file
I don't know if this is useful or not, let me know!