I have an Ubuntu set up (11.10) and 2 ATI Radeon HD 6870 cards. It detects one, but not the other for some reason when I try to run
When I run
lspci, it finds both cards.
lspci lists PCI devices.
./poclbm.py lists OpenCL devices. If the former finds the device but the latter does not, your second GPU must not be registering as an OpenCL device.
Ubuntu 11.10 has a notorious reputation for multiple-GPU bugs. (See here and here.) I personally have had issues with 11.10 mining that were resolved simply by switching to another release. I would suggest you instead use Ubuntu 11.04 (Natty Narwhal); it is generally regarded to be the most reliable Ubuntu version for mining.
If that is not an option, or installing 11.04 doesn't fix the problem, try the following steps:
aticonfig -f --initial --adapter=allagain. Make sure it runs without errors.
aticonfig --list-adapters. You should see two.
Those steps should fix any configuration issues. If your problem persists, it's either a hardware issue (or an Ubuntu 11.10 issue).