Is your monitor connected to the "Master" or "Primary" GPU card in the Crossfire configuration?
Eyefinity is compatible with Crossfire but uses only the Master/Primary GPU card as its output .
From Moderator of AMD Forums - amdmatt who wrote this FAQ: https://community.amd.com/docs/DOC-1318#jive_content_id_Is_AMD_CrossFire_compatible_with_AMD_Eyefini...
Is AMD CrossFire™ compatible with AMD Eyefinity?
Yes, AMD CrossFire and AMD Eyefinity are compatible. Since only displays connected to the primary (master) card are supported in AMD CrossFire, the number of displays that can be enabled in AMD Eyefinity mode is limited to the number of supported displays of the primary card.
If you would like further information regarding AMD Eyefinity setup please visit, www.amd.com/FAQ1.