The people suggesting two half-bar alnico magnets in a HB just because it's possible in P-90s, forget the most important factor in magnet making: MASS, as the resulting footprint-tone is directly proportional to its mass. In a P-90 mag mix-and-match works because two whole-size bars are used.
Two half mags joined together don't make up for one whole. So they actually sound as one pretty degaussed, not to mention the chaotic flux storm found on the joint points of the mags. This is even worse with spacer-sized alnico mags, and that's why you don't find'em listed on the usual p'up parts manufacturers and re-sellers.
There's at least one p'up manufacturer whose name I fail to remember using half mags in their products, but they're cut so they put an say A5 the three lowest strings and an A2 the three highest. This arrangement creates a permanent degaussing effect and a vortex in the way flux circulates, directly found on the two middle strings. Everything else is the same as a normal HB. I haven't personally tried any similar arrangement, mind you, however that's because just I KNOW it's a bad idea.
HTH,
