this has been solved somehow by now;
atleast for xfce4.12 i think it is related to the radeon-driver; after installing the firmware-linux-nonfree as suggested by debianwiki for "atihowto" , the second monitor was correctly recognized and dektop extension was normally activated.
i don't know how to achieve this correctly for devuan jessie (i tried to install fglrx-drivers for r5 230 and the monitors got recognized, it was however not possible to extend the desktop as the "field" was limited to a restritcted pixelarea)

$ xrandr 
Screen 0: minimum 320 x 200, current 2560 x 1024, maximum 8192 x 8192
VGA-1 disconnected primary (normal left inverted right x axis y axis)
HDMI-1 connected 1280x1024+0+0 (normal left inverted right x axis y axis) 359mm x 287mm
   1280x1024     60.02*+  76.00    75.02  
   1152x921      65.99  
   1024x768      75.03    70.07    60.00  
   832x624       74.55  
   800x600       72.19    75.00    60.32    56.25  
   640x480       75.00    72.81    66.67    59.94  
   720x400       70.08  
HDMI-2 connected 1280x1024+1280+0 (normal left inverted right x axis y axis) 359mm x 287mm
   1280x1024     60.02*+
   1152x864      75.00  
   1024x768      75.03    60.00  
   832x624       74.55  
   800x600       75.00    60.32  
   640x480       75.00    59.94  
   720x400       70.08  

So it can work, but I am not sure what to advise.

Geoff

thank you very much for this great distribution that i like very much!

This is my first post, but i have used Devuan now for some month and found it very good + responsive.

the problem occurred with a new installation of Devuan Jessie.
during installation i had two monitors connected to my graphics card (r5 230 hd radeon) and after installation i found that the display was mirrored on both, with no option to extend the desktop.
after some shallow research i decided to upgrade as it seemed to me that xfce4<4.10 would not allow for desktop extension on second monitor.

after upgrade to devuan ascii the same problem persists,
only one monitor is being recognized, but the content is mirrored on both displays, with no option to extend display.

this made me wonder because i have had an devuan ascii instalation before, where it would normally extend the desktop to second monitor.
at that time, during installation , most probably i had only one monitor connected.

xrandr gives the following output
xrandr
xrandr: Failed to get size of gamma for output default
Screen 0: minimum 640 x 480, current 1280 x 1024, maximum 1280 x 1024
default connected 1280x1024+0+0 0mm x 0mm
   1280x1024      0.00*
   1024x768       0.00 
   800x600        0.00 
   640x480        0.00

one monitor is connected with dvi to dvi, the other with hdmi to dvi.