randr/xinerama: mixing screens from different heads?

[Csillag Kristof]

Dear all,

I have a question about X config/randr/xinerama.

If I have two monitors connected to device 1 (1A and 1B),
and a third monitor connected to device 2 (2A),
how do I configure X to respect the physical layout of the monitors,
which is "1A-2A-1B"?

As you see, the monitors are mixed: the ones connected to the same
device are on the left and on the right.

"1A-1B-2A" would be easy (just put the screen 2 RightOf screen 1), but
for various reasons, I can not wire up the monitors that way.

Can I separate the two monitors from one head into two different screens?
How do I do that? (I have tried adding Screen numbers to the device
sections,
and fiddling with the PCI id-s, but it did not help.)

Device 1 is a FireMV 2200 PCI card (radeon driver), and
device 2 is a Radeon X1250 chipset (radeon driver).

Thank you for your help:

    Kristof