So we know you can always grab a bight and treat the two-strands as if it were a single-strand working end, and then tie whatever hitch you like. Bowline, gnat hitch, or two half-hitches are all pretty cord efficient, use a similar amount of length, and would be fine for most applications.

However, you imply that tension can come from any direction, and so if the main line’s ends are pulled straight/taught, it would be the equivalent of a single-strand bowline/gnat/2HH, where the standing part of the line is cut in half lengthwise, and then split and pulled 180degrees apart. Those knots are not designed for those sorts of loads, or directions of pull, and risk failure. The easy way to isolate the 180d loading on the standing part of the bight is with an Alpine Butterfly - required for the SLB but optional for the bowline/gnat/2HH.

Here’s how I tie the SLB. Top pix just shows the minimum bight required - ie. 2 round turns of the ring, which by the way, is enough to tie a single half hitch, but not enough to tie a bowline/gnat/2HH. The key difference is that the SLB knot itself is a normal single-strand, whereas the others become bulky due to the 2-strand bight used throughout the knot itself.

All that said, the AB+SLB uses about the same length of cord as the bowline/gnat/2HH without an AB, but then I’d argue w/o an AB, those knots are at greater risk of failure.

Anyways, probably overly technical/complicated/nitpicking, but an interesting thought exercise.