Very good idea to check that 2nd GPU core in fact computes correct results. I've already post here topic about problems with 5970 -- under Windows and Catalyst 10.4 & 10.5 2nd core of 5970 just producing pure garbage also working slower than 1st core (and 1st core at least producing correct results).
I have no idea how it's possible to miss such bug, so I have feelings that 10.4 & 10.5 wasn't tested at all with 5970 by ATI. There no such problems with 10.2 & 10.3
Multi-gpu issues are being looked-upon by developers. You can expect changes with one of the upcoming releases.
I don't have any problems with the second GPU on my HD 5970. I'm running Linux (Ubuntu 9.10) with Catalyst 10.3 and SDK 2.1
from OpenCl info alone? i do not think you can. with aditional API from ATI you can. but why? it is bug. you should just provide for selection of GPU for user.