The book that's been out on my desk recently contains excellent explanations of combination printing, including specific tips such as

After making the exposure of one part of the montage, pencil a few dots lightly just inside the outline of the already exposed parts (not the unexposed side otherwise you'll make white spots), then you can refer to the dots for positioning the second exposure, and rub the pencil off in developer with your fingers....

Use an ordinary "orange" camera filter on the enlarger, not because it's safe, but because it's brighter than the red filter, and if you work quickly, it's "safe enough" while you position your paper for the second exposure....

Lootens on Photographic Enlarging and Print Quality, by J. Ghislain Lootens F.P.S.A., F.R.P.S. 1944