patch applied upstream.

will be fixed by rebase

I can reproduce with: virt-viewer-0.5.7-7.el7.x86_64

VERIFY:
Steps:
Prepare a client has two physical monitors.

1. # virsh start demo
# virsh dominfo demo
Id:             3
Name:           demo
UUID:           4cd561bf-ba7f-404c-9bfa-2651534c56dc
OS Type:        hvm
State:          running
CPU(s):         2
CPU time:       61.2s
Max memory:     1048576 KiB
Used memory:    1048576 KiB
Persistent:     yes
Autostart:      disable
Managed save:   no
Security model: selinux
Security DOI:   0
Security label: system_u:system_r:svirt_t:s0:c246,c937 (enforcing)

2. 
Add following content to ~/.config/virt-viewer/settings
[4cd561bf-ba7f-404c-9bfa-2651534c56dc]
monitor-mapping=1

3. # virt-viewer demo , make sure only 1 display is shown.

4. # virt-viewer demo -f
One display comes out fullscreen.

5. Click "Leave fullscreen" button on the top of virt-viewer
And open another display, "View" -> "Displays" -> "Display 2"

Result:
step5, display 2 comes out in window mode. 

As the result, VERIFIED

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://rhn.redhat.com/errata/RHBA-2015-0295.html