Best practice is to have a permanently attached lanyard at the boom end, stowed along the underside of the spar, so that it is accessible from inboard even when the boom is fully out. A spliced loop at the end can be attached to a preventer line on one side and then tacked to a line on the other side after a gybe, without crew having to go forward of the mast as would be necessary if only one line were used.
The membership association for owners of Dufour Yachts