I don't have room for 2 enlargers, so I'developed my own system using an old diffuser filter from an old slide copier. For pre-flashing I just do a simple test strip. I usually use the exposure just before the one that shows a visible tone. If I want to give an overall slight fog I use the 1st visible value - I find this works quite well with snow scenes - particularly if I want to tone the print.

For post-flashing I do the usual test strips (and probably a work print for comparison). I then print a 6x4in section of the print containing the 'problem' highlights. I then run use that as a test strip for post flashing.

The beauty of using a diffuser is that you don't have to keep moving the paper or changing the lens aperture.

For pre-flashing I find it useful to keep a record of exposures for different papers to avoid having to do test strips.

Having said all that, I only use pre or post flashing on graded papers, I find split-grading more effective with VC papers.