On a Citrix XenApp/Presentation Server install, I am unable to split my published desktop, nor my published apps. My published desktop is not spanning across both monitors. How can I fix this?

This can occur if the session resolution is restricted, perhaps due to session memory not being enough. Go to the Citrix Farm Properties and make this change:

ICA Settings
Maximum Memory to use for each Session's Graphics
8192 kilobytes

The 8192kb suggestion will work for two 1600x1200 monitors with 16-bit color. If you have higher resolution or color depth, you need more memory. You can calculate the memory required using the formula below:

(colordepth/8) * horizontal-resolution * vertical-resolution * n

Colordepth is in bits (eg, 16 or 24) and n is the number of monitors. The horizontal and vertical resolution if for one monitor (eg. 1600 and 1200).

Higher Resolution monitors or more monitors
The Citrix KB article at http://support.citrix.com/article/CTX114497 provides a way to increase the session memory on Windows Server 2003. Apparently, Citrix supports up to 16MB – which is still lesser than the memory you need for 1680x1050 resolution at 24 bit depth but would work if the color depth is reduced to 16 bit.

To use 16MB session memory, change the following registry key:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Terminal Server\Wds\icawd\thin16
Set "MaxLVBMem"=dword:01000000

The forum post at http://www.brianmadden.com/forums/t/12705.aspx has a user reporting that he got an array of 8 monitors working by increasing the value to 32 MB with some mixed results: “You can increase that MaxLVBmem key to 32 MB because we've done it and it works, well, kind of. We've seen some odd performance issues and unusual disconnects with further testing.”.

We have users who have reported that increasing the maximum memory beyond 8192kb does work for them. However, this may not be supported by Citrix.

If the above does not help, check if your default.ica file does not have an incompatble setting. Steps for this are at http://www.splitview.com/node/66