RandR vs ScreenLayout

[Ralf Mattes]

Hello list,

I'm trying to configure a desktop system with three monitors attached to two graphic 
cards (Radeon HD 4200 and Radeon HD 4650). 
Somehow I seem to be stuck between two suboptimal/half-working solutions.

1) keep an empty xorg.conf - that way the server does provide xinerama information (via
    xrandr , I assume), do the window manager receives propper screen information.
    Unfortunately, with this setup the X server only starts on graphic card.

2) configure both  devices and a 'ServerLayout' section in my xorg.conf. Now, all three monitors
     work and I get an X session with two screens. But unfortunately the server stops to report 
     xinerama information which causes all sorts of problems with the window manager. And 
     I can't even query the number of active screens via xcb.

Is there any way I can configure my server to activate both graphic cards AND get propper xinerama
information? (N.B.: activating xinerama in the xorg configuration causes the server to segfault ...).

Here's some version info:
 X.Org X Server 1.19.0
 Release Date: 2016-11-15
 [ 15122.246] X Protocol Version 11, Revision 0
 [ 15122.246] Build Operating System: Linux 3.16.0-4-amd64 x86_64 Debian

TIA, Ralf Mattes