Thinking of handcuffs... I was thinking that maybe there is a knot that would self-lock sufficiently well not to require locking half-hitches or the like.
Here is an idea: since the HFP Slippery 8 Loop starts from a Fig. 8 stopper, which is symmetrical, a second splayed loop can be formed by weaving the S-Part (now the second tail) back into the Fig. 8 from the other side, in the same fashion...
That's in theory, but it doesn't seem to work too well for me... I think that with two loops, pulling in opposite directions the Sippery 8 U-turn toggle on which it relays may not work for either one. Maybe there is some other construct that would toggle-lock both loops better?
p.s. I took the liberty of blatantly copying
Roo's image for the background of this sketch.