personally, I would not put two different sized rims on a bike cause I feel it gives a very unbalanced unstable feel but then again, I dont dirt jump very often so it may improve that but I'd stick with either 2 24's or 26's for everyday riding.
I just bought a 2006 Specialized P.1. It has a 26" wheel in front with a knobby tire. The rear is 24" with a regular tread tire. I have only rode it in snow but it handles better than the Haro x24 which is a BMX cruiser- 24" wheels with semi-knobby tires. The Haro has more traction though. So maybe the tire type is more important than wheel size for traction. As far as jumping goes I don't know which one is better.
Most mountain bikes use 26 in (559 mm) wheels, though some models offer 24 or 29 in (520 or 622 mm) wheels(this size is for road/hybrid bikes only not mountain bikes).Inch-sizes for bicycle wheels are not precise measurements: a 29 inch wheel is actually a 700c wheel, which has a 622 mm (24.48 inch) bead seat diameter. 700c is an archaic designation that persists because of its early popularity, though the diameter is 700 of no actual unit of measurement. In some countries, mainly in Continental Europe, 700c wheels are commonly called 28 inch wheels, nevertheless these wheels are equivalent to 29 inch wheels in the English speaking countries.
24 inch wheels are used for Dirt Jumping bikes and sometimes on freeride bikes, rear wheel only, as this makes the bike more maneuverable. 29 inch wheels were once used for only Cross Country purposes, but are now becoming more commonplace in other disciplines of Mountain Biking, and are predicted to be the choice wheel size on Downhill Bikes within a few years by some.
Wheels come in a variety of widths, ranging from standard rims suitable for use with tires in the 26 in x 1.90 in to 2.10 in (559 x 48 to 53 mm) size, to 2.35 and 3.00 in (60 and 76 mm) widths popular with freeride and downhill bicycles.
Yes,the P.1 is a single speed dirt jumper. And built with a shorter chainstay than the other P bikes. So the 24" wheel design is for a specific purpose. I believe Kona makes,or did make a dirt jump bike with 24" wheels,but using the same frame as the bike with 26" wheels.
I have not yet used the Haro x24 or P.1 for dirt jumping so I can't comment on that. I use the Haro primarily as a SS commuter on roads and dirt paths and just got the P.1