I did experience the same with 2xHD5870 i had on water, i ended up flashing the second card with a copy of the first cards bios, there was no issue doing so as they where both the same card, reference models and there where no differences apart from the second cards clocks and voltages.
Once i flashed my second card i could control clocks and voltage on both cards but if i remember rightly i had to set same voltage for both cards which was a shame as the second card didnt really need the volts i had to give it.
Bios flashing is not easy if you have no experience doing so and it gets real tricky when your not flashing ref models as well.
If you went down that route id spend a few days reading up and taking advice on the whole process as you can end up with no video signals on your GPU if the flash goes wrong, you would then end up doing a "Blind flash" which is so difficult but can be avoided by having a spare card to pop in slot 1 if you "brick" a card with bad flash.
Always make backups of original bios first, just in case it goes wrong or you wish to sell them on and return them to stock bios, its literally a 2-3 second fool-proof process using GPU-Z
Also ask yourself....is it worth overclocking them? CF6870 is not to be sniffed at, some decent GPU power there even at stock clocks