Ok, ... so, let me see if I understand correctly- ¿the card won't support a 30" display when combined with another display, because of the resolution... so it needs either a second graphics card or a dual-link MDP adapter...? or is it because the MiniDisplayPort capabilities?
One 30" display in combination with any other smaller display will work fine, however
two 30" displays will require you to purchase a dual-link DVI apdator for the Mini DisplayPort port.
Which is fine, but the dual-link DVI adaptor (which is distinct from the
single-link DVI adaptor you already own) is both expensive and unreliable. So,
my point is that if you'd like to use two 30" displays then you'd be better off buying a second graphics card than the expensive, unreliable adaptor.
I still don't understand what causes this issue... is it present only when there's an Apple Display involved (eg. 24"LED)... What if NONE of the displays are Apple? would the card then support a 30" display(s) in any combination- for example a 30" to the DVI and a 24" to the MiniDisplayPort (none of them being Apple)?
So, here's the overview. Apple's new graphics cards ship with two ports: a dual-link DVI port and a Mini DisplayPort. So, without any adaptors, you can run a new 24" Apple Studio Display and a 30" LCD. With the relatively inexpensive single-link DVI (this is what you have from your iMac) or VGA adaptors you could substitute a non-Apple display of
up to 1920x1200 resolution.
But here's where the trouble starts to crop up. If you want to run two of anything with a resolution higher than 1920x1200 (say, dual 30"s or a 30" and a
Samsung 2343bwx) then you'll need to use Apple's ridiculously overpriced and horribly unreliable dual-link DVI adaptor. Which means you're probably better off getting a second graphics card instead of the adaptor.