I don't see how to typeset a multilingual text parallel by page strictly within groff. However, suppose
1. the input comes from two different files that contain synch points. 2 corresponding synch points must appear on facing pages. 3. segments between successive sync points always fit on one page. Then a small non-troff program can merge synchronized segments alternately from the two files. To process the merged file, direct segments by pairs into diversions named, say, A1, B1, A2, B2, ... When the sums of lengths of either Ai or Bi overflow a page, emit the diversions in order A1, A2 ... .bp B1, B2 ... .bp. Rename the overflowing pair A1, B1 and continue. Synch-point macros end, measure, and start diversions, invoking an overflow macro as necessary.
