Cutting extruded heatsink
I would not want to try it. Although the idea is reasonable from a thermal point of view, mechanically it would be very chancy. The problem is getting good contact over your 4 cm hot spot. With a single heat sink, putting even clamping pressure on the four corners will make a reasonable contact with the hotspot, assuming the upper surface of the hotspot is parallel with the board surface.

With two pieces of heatsink, the hotspot is off-center for each piece. Trying to get good clamping pressure without tilting the heatsinks and ruining the hotspot/heatsink interface would be fairly tricky.