"Factorial Development" is a technique where you choose your paper development time from how long it takes the first shadows to appear in the print. Say you see shadows just coming up at 10s (typical for properly exposed RC paper), you would need to develop for about a minute. If it takes 20s for shadows to appear (typical for properly exposed FB paper), you develop for two minutes; i.e. the total development time is 6x the time required for shadows to appear. Going to 9x is considered acceptable and in some cases desirable because the development activity slows right down; the longer development can be more even and make sure you get a real black. You should never develop in trays for less than 1 minute for reasons of uniformity.

If you get a completely black image from the Kentmere neg with only 2s exposure, yet you can print fine from HP5, it would seem that your K neg is very thin (underexposed in camera, or underdeveloped). If that is the case, you'll probably need to print at a high grade (try Grade 4 or Grade 5 if using the Ilford filters; Grade 5 is 1 stop slower than the rest) with very short exposures and small apertures - stop the enlarger lens down to f/32 or something. That might get you a manageably short exposure. If the print is still too dark even with a 2s exposure, try putting a neutral density filter in with the multigrade filter.