Part I Linux:
On many Linux distribution you can easily add more than one language and switch among them with a keyboard shortcut, usually set Left Alt + Shift.
If you want to physically install 2 keyboards you can follow what suggested in Two keyboards on one computer...
First you need to identify the Device ID e.g. with xinput
xinput -list | grep -i key
Then you can set the layout using setxkbmap with something like
setxkbmap -device <NUMBER OF 2nd DEVICE> gr
Some references:
- Always
man xinput and man setxkbmap
You can use something more complex too: from here e.g.
setxkbmap 'pl,gr' -variant ',polytonic' -option 'grp:alt_shift_toggle'
Part II Windows:
If you do not want to Add an input language as is suggested since from Vista
You can edit documents in multiple languages by changing the language (the input language) in which you type. Input languages are included with Windows, but you need to add them to your list of languages before you can use them.
in another question of SE, Using multiple keyboards with different keyboard layouts in Windows, it is suggested this program RightKeyboard.
On some forum you can find proposed to use HID macros a software for sending keyboard macros activated from different USB devices...
If you want instead to follow a tutorial on how change keyboard input language
you can find interesting the link above.
Note that even here you can switch between 2 language layout as easy as to press Left Alt + Shift.
Reference:
PS> A less then 4$ and some time alternative solution: transparent stickers for keyboard to mark the Greek letters you want on the non Greek keyboard. Somehow an hardware solution too :-)