FOTR is an easy stitch-together, actually, as the audio and video both end at the cut to black. Haven’t checked that part of 44rh1n’s release to be certain that’s what he did, but I don’t see why it’d be any different (at work so can’t check now). [JEDIT: Yep, the disc change point looks and sounds exactly as it should in 44rh1n’s version, at least on the “V3” single 44GB file version. I’d expect the smaller “V1” single 22GB file version to be the same.]
TTT and ROTK, however, have the audio continuing on after the cut to black, playing over the “insert disc 2” screen, so they require an L-cut, meaning you have to overlap the disc 1 audio with the beginning of the disc 2 audio, which requires audio reencoding.
I’ve got audio tracks for TTT and ROTK that are properly stitched together like this that someone made for kk650’s regrades and shared on tehparadox, but I can’t recall if they’re lossless DTS-HD, regular DTS, or AC3, or whether they’re 6.1 or 5.1 - but if you’re interested I can share them. They should work with any stitch-together of TTT and ROTK that just join the two at the cut (so there would be no black screen whatsoever between the last frame of picture on disc 1 and the first frame of picture on disc 2).