You have discovered the dirty secret of many Kniphofia cultivars; the bloom season can be pretty short. I would expect that 2 weeks blooming is on the low side of what you should expect, 6 weeks is more normal if the plants are getting the conditions of sun, water and nutrients they prefer. Unseasonal heat waves at time of blooming can also impact the duration, as most Kniphofias are high elevation plants from the Drakensberg Mountains that are adapted to cooler summer conditions and moist soils in summer, not our Pacific Coast summer long drought.

I know of no cultivars that rebloom with deadheading, but there are cultivars that bloom over a longer season than others, and different varieties can be selected to stagger the bloom season. Kniphofia 'Winter Cheer' is a popular winter blooming cultivar here in northern California, but winter blooms probably aren't as practical with PNW winter rains and winds.

I's suggest looking around you to see which cultivars do best/bloom longest in your local conditions, and stick with those.

I have Kniphofia thompsonii var. snowdenii that I bought a couple of years ago from Joy Creek nursery. After being divided down to a single 4" pot in April it is now filling a 2 gallon pot, is halfway through a nice bloom spike and is sending up a second. This will bloom into October.