I cannot tell whether you are on windows are unix as you have mixed syntax. On unix, parens need escaping by \ and lines are continued with \. On windows the parens don't need escaping and the line continuations are set by ^. So the following assumes you are windows, though I am better with unix syntax.
try using in memory images mpr: to avoid potential problems with nested parens with -clone and -crop (needed since one cannot delete the first image until the end with clones, whereas with mpr, it can be deleted right away). Most of the time clones works just fine and is easy. This case is a bit complicated and so it seemed to me that mpr might be simpler.
convert ( input.jpg -strip -colorspace Gray -filter Lanczos -resize 800 -unsharp 1.1x1.0+1.0+0.05 -write mpr:tmp +delete ) ^
-size W1xH1 -depth 8 canvas:white ( mpr:tmp -crop W2xH2+X1+Y1 +repage ) -geometry +X2+Y2 -composite ^
( mpr:tmp -crop W2xH2+X3+Y3 +repage ) -geometry +X4+Y4 -composite -quality 92 output.jpg
or use -flatten which is cleaner and simpler when multiple images are to be composited
also you can remove the crop to make it easier
Window:
convert ( input.jpg -strip -colorspace Gray -filter Lanczos -resize 800 -unsharp 1.1x1.0+1.0+0.05 -write mpr:tmp +delete ) ^
-size W1xH1 -depth 8 canvas:white ^
-page +X2+Y2 mpr:tmp[W2xH2+X1+Y1] ^
-page +X4+Y4 mpr:tmp[W2xH2+X3+Y3] ^
-flatten -quality 92 output.jpg
Unix:
convert \( input.jpg -strip -colorspace Gray -filter Lanczos -resize 800 -unsharp 1.1x1.0+1.0+0.05 -write mpr:tmp +delete \) \
-size W1xH1 -depth 8 canvas:white \
-page +X2+Y2 mpr:tmp[W2xH2+X1+Y1] \
-page +X4+Y4 mpr:tmp[W2xH2+X3+Y3] \
-flatten -quality 92 output.jpg
see
http://www.imagemagick.org/Usage/layers/#flatten
http://www.imagemagick.org/Usage/files/#mpr
One would have to see your images and exact command line to advise further about quality.
Note the +repage after a crop to remove the virtual canvas, though this is not necessary for jpgs
Sorry I seemed to have posted just about the same time as el_supremo and we both have about the same solution.