OK. Necro thread, but the law of physics you're referring to is equivalent resistance with parallel resistors. It states the 1/Req = 1/R1 + 1/R2 + .... + 1/Rn. This means if you have 2 8 ohm loads in parallel (which is how two 8 ohm cabs would be) you have 1/8 + 1/8 = 1/?. So it's a 4 ohm load. HOWEVER, the amp has a minimum load of 4 ohms to each side. This means you need a 4 ohm load on the left AND on the right for max power output. So, if you have a 412 with 16 ohm speakers, wire them all in parallel, and you've got 1 4 ohm cab. Otherwise, if you have 8 ohm speakers, you can do two jacks. Each jack has 2 speakers connected in parellel, giving a 4 ohm load. Best option for 2 cabs is 2 4 ohm cabs, or 2 cabs with double jacks, each jack at 8 ohms. Tube amps and solid state amps also have no problems going above the minimum ohm requirement. You just get less output. For a GX130C, for instance, you get 95 W per side @ 4 ohm, but 65 W per side @ 8 ohm. This is why tube amps and their hardcore output transformers are so awesome, because you can set them for 4, 8 , or 16 ohm output and it's always the same wattage.
