That is not possible. You must restore within the same FlexVol:
snap restore -t file -s test -r /vol/chris2/test.txt /vol/chris1/test.txt
WARNING! This will restore a file from a snapshot into the active filesystem. If the file already exists in the active filesystem,
it will be overwritten with the contents from the snapshot.
Are you sure you want to do this? y
You have selected file /vol/chris1/test.txt, snapshot test
It will be restored as /vol/chris2/test.txt
Proceed with restore? y
snap restore: restore_as_path and restore_from_path have to be specified in the same volume.