I read his post and he suggested using one register set for user mode and one for interrupt mode, purely to avoid the movem.l overhead when processing interrupts & not actually run code using the two register sets simultaneously.
Quite frankly, what does your interrupt process that you need to worry about movem? If that's one cycle per instruction, and you save at most 15 registers, then that's 30 cycles maximum for the movem. Now, what does your interrupt do that it requires less than 30 cycles in actual payload code?
Remember, we're talking here about a 100MHz CPU. Not a 1.79Mhz 6502 which, I admit, did run into such problems.