I have done this in the past with Potassium Ferricyanide and hypo, I think, it's been a long time.
Better solution for the larger problem: Learn how much the paper "dries down" (tendency of highlights to be slightly darker after drying) and adjust exposure to compensate. My personal solution is to use a fairly low wattage bulb to evaluate the print after fixing (maybe 20-40w), in a fixture set into the ceiling above the viewing place, just behind the fixer tray (I never make a judgement in the tray, always looks too light). I drain the print off a corner for a few seconds, place it on a piece of plexiglass at an angle, step back, and view with only this one light on in the darkroom. (I also shield my eyes from direct light coming from the fixture.) After experimenting with the right wattage and distance of the light from the print, I never had the problem again. I probably have drying down, but I'm compensating in the viewing condition of the wet print.