I never even bother try doing x2 1B:B, since the 2nd jf is so hard to buffer, at least for me. However, x2 1B:B 2143b:B will always hit if you're near a wall/edge, care with w! though. + it's easier on big chars in open areas, it will most likely hit anytime then.
Tip: use 44B+K BE after x2...