Recently we started using Qubes OS 3.2, at some point we needed to enable the Greek language keyboard for a Fedora 23 based VM.
To avoid making permanent change to the VM we used the following command that enables the US and GR keyboard layouts while switching is made available by pressing the two shift buttons at the same time.
setxkbmap -layout "us,gr" -option "grp:shifts_toggle";
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