You need at least one monitor or dummy plug per rig if using the same GPU's
Without this the cards are not "connected to the system" so you will not see temperatures or be able to make changes to core voltages etc.
i noticed some interesting thing here: my 7XXX cards work if you select "connect to vga anyways" in display properties (and so do all newer cards in that rig after enabling it with that trick), whereas my 2XX cards dont offer to even try vga connectivity (when clicking "detect")
not sure why though, they are using same drivers