I came up with a solution that seems to work pretty well. I'm not sure if there's a better way, but I wrote a script that starts my VM and then monitors the clipboard on display :0 for changes. When a change is detected, it copies the clipboard contents to display :1. It does this bidirectionally, so I can copy and paste from the VM just fine too.

Here's the script:

#!/bin/bash



virtualbox --startvm "Windows 7" --fullscreen &

waitpid=$!



watch_clip() {

  local curr="" prev="" from=$1 to=:0



  # On first run, we need to copy from :0 to :1 but not vice versa

  if [[ "$from" == ":0" ]]; then

    xclip -o -selection clipboard -d :0 2> /dev/null | xclip -selection clipboard -d :1

    to=:1

  fi



  while true; do

    # Get the current clipboard contents

    curr=`xclip -o -selection clipboard -d $from 2> /dev/null`



    # Compare to previous results and copy if it's changed

    if [[ "$curr" != "$prev" ]]; then

      echo "$curr" | xclip -selection clipboard -d $to

    fi



    prev="$curr"   

    sleep 0.5

  done

}



watch_clip :0 &

watch_clip :1 &

wait $waitpid

Then all that's needed is the command to start the second X session:

startx ./.startwin7 -- :1

I haven't noticed any issues, but if anyone can think of a better way I'd definitely appreciate the input.